apple-internals/internals.txt

AA	Apple account
AAT	Apple Advanced Typography; font format and rendering engine
Accounts	launchd service: com.apple.accountsd; /System/Library/Accounts
ACDE	Apple Connect Device External? ACDEClient.framework, old two-step verification, derived from a company-internal AppleConnect system? server: appleconnect.apple.com
ACFS	Apple Clustered File System; deprecated file system for Xsan; acfs.framework
Acoustic ID	Siri feature to recognize songs
Action	extension type for quick interaction with foreign content within a host app; extension points: com.apple.services, com.apple.ui-services
Activity	jobs, coarse-grained work units of applications; tracked by the system across XPC, bears a QoS class for scheduling; low-level mechanism not to be confused with User Activity
AE	Apple Events; messaging system to invoke application functionality; CoreServices.framework/AE.framework; launchd services: com.apple.coreservices.appleevents, com.apple.AEServer (AE over network)
AGC	Apple Graphics Control, management of multiple displays and display port connections; launchd service: com.apple.displaypolicyd
ALF	Application-Level Firewall, launchd service: com.apple.alf (socketfilterfw)
Alloy	substrate for communication between user devices over Bluetooth and devices to iCloud, implemented over IDS; /System/Library/IdentityServices/ServiceDefinitions; launchd service: com.apple.identityservicesd
ALS	Ambient Light Sensor, AmbientDisplay.framework
Amber	Swift UI; SwiftUI.framework
AMFI	Apple Mobile File Integrity, checks code integrity based on code signature, stronger enforcement with hardened runtime, validates entitlement restrictions; launchd service: com.apple.MobileFileIntegrity (amfid), invoked by kernel through host special port 18
AMP	Apple Media Protocol? former parts of iTunes for iPod and iOS device access in Finder, Home Sharing; AMPDevices.framework, AMPSharing.framework; launchd services: com.apple.AMPDeviceDiscoveryAgent, com.apple.AMPDevicesAgent, com.apple.amp.mediasharingd
Anisette	two-factor authentication creates security codes on trusted devices using TOTP, probably using Circle keys, checked by HSA; AuthKit.framework; launchd service: com.apple.akd
AOS	Apple Online Services? historical name for iCloud
APNS	Apple Push Notification service, server infrastructure for remote push notifications over a single connection, clients subscribe to push topics, can be authenticated by app (remote notifications), device (Find My …), or Apple ID login (DSID); credentials in apsd keychain; launchd service: com.apple.apsd; server: courier.push.apple.com
App Nap	quiescence detection for applications and corresponding self-demotion in scheduler parameters, implemented within the application by frameworks, listens for occlusion notifications from WindowServer
App Sandbox	Seatbelt-based sandbox for apps; /System/Library/Sandbox/Profiles/application.sb; enabled with com.apple.security.app-sandbox entitlement; command line tool: asctl; launchd service: com.apple.secinitd
APRR	Access Protection Range Register? ARM CPU register to downgrade actual permissions of memory pages; used for JIT protection and by AMFI to freeze user code after checking
APFS	Apple File System; copy-on-write file system with support for volume space-sharing, per-file encryption, and snapshots
APT	Adaptive Picture Timing? ProMotion; dynamic screen updates with 120Hz base frequency
ASL	Apple System Logger, superseded by Unified Logging; /etc/asl; stored in /var/log/asl; launchd service: com.apple.syslogd; command line tool: syslog
ASR	Apple Software Restore; restore entire volumes from sources like disk images (HDI, SIU), also restores based on APFS snapshots and snapshot deltas; command line tool: asr
Assertions	power state management allowing applications to prevent sleeping; launchd service: com.apple.powerd; command line tools: caffeinate, pmset
Assessment	checking of System Policy; term used on iOS for school exam apps that lock the device
Asset Cache	discretionary caching server for Mobile Assets, Packages, iOS updates, App Store content, ODR, MMCS data; launchd services: com.apple.AssetCache.builtin, com.apple.AssetCacheLocatorService, com.apple.AssetCacheManagerService, com.apple.AssetCacheTetheratorService; command line tools: AssetCacheLocatorUtil, AssetCacheManagerUtil, AssetCacheTetheratorUtil
Assistant	Siri; dictation and semantic understanding, Intent is communicated to and enacted on the client, uses TTS; /System/Library/Assistant, AssistantServices.framework; server: *.siri.apple.com
ATS	App Transport Security, sandbox mechanism only allowing TLS-secured connections
ATSUI	Apple Type Services for Unicode Imaging; rendering engine superseded by CoreText.framework, font management; ApplicationServices.framework/ATS.framework; launchd service: com.apple.fontd; command line tools: atsutil, fontrestore
Authorization	discretionary access control policies for high-level services; similar to PAM; policy stored in /var/db/auth.db
Avatar	Memoji; AvatarKit.framework
AVB	Audio Video Bridging, low-latency audio over Ethernet; launchd service: com.apple.avbdeviced; command line tool: avbdiagnose, avbutil
AWD	Apple Wireless Diagnostics, sends system telemetry to Apple; CoreAnalytics.framework, WirelessDiagnostics.framework; launchd services: com.apple.awdd, com.apple.analyticsd
AWDL	Apple Wireless Direct Link; secondary WiFi interface that runs in parallel to an active WiFi access point connection, similar to WiFi Direct (p2p interface), uses a randomized MAC, used for peer-to-peer networking: AirDrop, AirPlay; DeviceToDeviceManager.framework
Bezel	on-screen overlays for hardware volume buttons, screen brightness, Bluetooth HID, and others; /Library/Application Support/Apple/BezelServices, launchd services: com.apple.loginwindow, com.apple.OSDUIHelper
BOM	Bill of Materials; format to store contents of installer Packages; command line tool: lsbom
Bonjour	mDNS; launchd service: com.apple.mDNSResponder.reloaded; command line tool: dns-sd
Boot Cache	disk cache pre-heating at boot time with typically loaded applications; /var/db/BootCaches; launchd service: com.apple.warmd
BPR	Boot Progress Register; set-only flags to track boot mode (normal, DFU, recovery), part of Keybag class key derivation within SEP, so passcode-protected keys are inaccessible in DFU and recovery
Bridge	T2 ARM CPU in Intel Macs to drive Touch Bar and Secure Boot, runs a derivative of watchOS, boots the platform and the Intel CPU, communication from macOS uses RemoteXPC, which uses HTTP/2 over a USB-Ethernet interface; launchd service: com.apple.multiversed; /System/Library/MultiversePlugins; command line tool: remotectl
Bulletin Board	application push notification management, aggregates local and remote push notifications; BulletinBoard.framework
Cache Delete	cleanup for various caches; /System/Library/CacheDelete; launchd service: com.apple.cache_delete (deleted)
CAML	Core Animation Markup Language; XML file format for layers, shapes and animations
Carousel	derivative of SpringBoard for Watch home screen, watch face, and notification center
Celestial	media streaming used by ReplayKit for game broadcasts; Celestial.framework
Certificates	validity checked using OCSP stapling, locally installed CRLs, and transparency logs; /System/Library/Security/Certificates.bundle; launchd services: com.apple.trustd, com.apple.trustd.agent, com.apple.ocspd
Circle	cryptographic primitive to exchange public keys of all trusted devices of one user, signed by all Circle peers; used by SOS; command line tool: tpctl; KeychainCircle.framework; iCloud identity keypair as an additional Circle peer, triggers countersigning from all trusted devices, private key synced across all trusted devices, new devices can pull this key from Secure Backup to join the Circle
CKKS	CloudKit Key Store, backend for SOS and Circle using CloudKit, currently stores the ApplePay, AutoUnlock, Engram, HealthKit, HomeKit, and Manatee PCS keys; secrets are unknown to Apple; launchd service: com.apple.secd; command line tool: ckksctl
Classroom	launchd service: com.apple.studentd
Cloud Pairing	part of Alloy, Bluetooth out-of-band pairing over iCloud for Continuity; launchd service: com.apple.cloudpaird
CMAS	Commerial Mobile Alert System, now known as Wireless Emergency Alerts (WEA)
Commpage	user-mapped kernel data, like vdso/vsyscall on Linux; mapped at 0x7fffffe00000
Communications Filter	recipient blocking for iMessage, FaceTime, Mail; launchd service: com.apple.cmfsyncagent
Companion	iPhone that is paired with Watch; communication uses Alloy over Bluetooth
Continuity	umbrella term for Handoff, Sidecar, SMS relay, Universal Clipboard, Watch unlock, WiFi call relay and others; SMS relay works by proxying to iMessage, other services use Alloy
CPML	CorePrediction Machine Learning; CPMLBestShim.framework
CRD	Conference Room Display; Apple TV mode
CSR	Code Security/Signing Restrictions/Requirements? also called System Integrity Protection (SIP) or rootless mode; collection of kernel-level security restrictions regarding file system modification, unsigned Kexts, Taskport access, NVRAM access, DTrace; /System/Library/Sandbox/rootless.conf; command line tool: csrutil, rootless-init
CTK	Crypto Token Kit; smart card management, also for the secure element on iOS? launchd service: com.apple.ctkd; command line tool: sc_auth
CTS	Centralized Task Scheduling; execution of DAS tasks; /System/Library/UserEventPlugins/com.apple.cts.plugin
CVMS	Core VM Server/Service? compilation of GPU shaders; launchd service: com.apple.cvmsServ
DAAP	Digital Audio Access Protocol; used by Home Sharing (with Rapport token) and by the Remote app to control Apple TV (with pairing token); payload unencrypted; DAAPKit.framework; Bonjour services: _atc._tcp, _home-sharing._tcp, _mediaremotetv._tcp, _touch-able._tcp
DAS	Duet Activity Scheduler; scheduling policy engine behind NSBackgroundActivityScheduler and XPC activities; /System/Library/DuetActivityScheduler; launchd service: com.apple.dasd
Data Detectors	text analysis to highlight phone numbers, street addresses, and the like; DataDetectors.framework
DataVaults	directories with the UF_DATAVAULT special flag; read access limited under CSR
DAV	Distributed Authoring and Versioning; network protocol on top of HTTP for syncing calendars (CalDAV), contacts (CardDAV), and formerly also bookmarks (BookmarkDAV)
DCIM	Digital Camera Images; DCIMServices.framework
DEP	Device Enrollment Program; devices check in with Apple during Setup Assistant to query for their enrollment status, retrieve MDM server URL to fetch initial configuration profile
DFR	Dynamic Function Row?, TouchBar; /System/Library/CoreServices/ControlStrip.app; DFRFoundation.framework
Differential Privacy	crowdsourcing without user tracking; privacy budget for management of anonymity set; used for keyboard words, emoji, Spotlight searches, Parsec deep links, HealthKit usage, Safari telemetry; /System/Library/DifferentialPrivacy; stored in /var/db/DifferentialPrivacy; launchd service: com.apple.dprivacyd
DND	Do Not Disturb
DSID	Destination Signaling Identifier, unique ID for IDS login on a specific device
DTrace	system-wide tracing infrastructure, command line tools: dtrace, *.d, dappprof, dapptrace, dtruss, errinfo, execsnoop, fddist, fs_usage, imptrace, iopattern, iopending, iosnoop, iotop, lastwords, latency, opensnoop, plockstat, rwsnoop, sampleproc, sc_usage, topsyscall, topsysproc
Duet	telemetry collection engine for system and user events, forecasting by machine learning, backend for DAS, Proactive, Relevance, Screen Time, thermal and battery management; /System/Library/DuetKnowledgeBase; CoreDuet.framework, CoreKnowledge.framework, CorePrediction.framework; launchd services: com.apple.coreduetd, com.apple.knowledge-agent
Dyld Shared Cache	dynamic linker cache, stores all system libraries in prelinked form, original library files are removed; /System/Library/dyld; command line tool: update_dyld_shared_cache
EAS	Exchange Active Sync; network protocol for accessing Microsoft Exchange servers
Energy Impact	unitless metric for per-application energy consumption, machine-specific coefficients; /usr/share/pmenergy; launchd services: com.apple.sysmond, com.apple.thermald
Engram	Messages in iCloud; devices store received iMessages in CloudKit; Engram.framework
Entitlements	capability-like attributes bound to executables by code signing; some entitlements like App Sandbox restrict ambient authority, some gradually relieve those restrictions (using Seatbelt), some services or system calls grant privilege based on caller entitlements
ESS	IDS user directory, public key distribution for iMessage and CloudKit sharing, uses Transparency; server: *.ess.apple.com; launchd service: com.apple.identityservicesd
FaceTime	video calls, employs the ICE (establishing peer-to-peer connection), STUN (session credential exchange) and SRTP (encrypted media streaming) protocols; FTServices.framework; launchd services: com.apple.videoconference.camera (avconferenced)
FairPlay	DRM system used by app and media stores; CoreADI.framework, CoreFP.framework; launchd services: com.apple.adid, com.apple.fpsd; credentials stored in /var/db/fpsd
Family Circle	Family Sharing; launchd services: com.apple.familycircled, com.apple.askpermissiond
FDE	Full Disk Encryption, FileVault; command line tool: fdesetup, sysadminctl
Feldspar	Apple News; Silex.framework
FiDES	Fi? Distributed Evaluation Service? ingests and aggregates Differential Privacy data for unlinkability? used for emoji, Suggestions, Dictation; /System/Library/DistributedEvaluation; DistributedEvaluation.framework; server: fides-pol.apple.com
Find My …	location sharing by explicitly querying devices remotely or collateral beacon detection using Search Party; launchd service: com.apple.icloud.fmfd (find my friends)
Firmlink	bi-directional non-symbolic link between the read-only system volume and the data volume, additional symlinks and mountpoints in the root directory are virtually allocated; /usr/share/firmlinks, /etc/synthetic.conf
FollowUp	user interaction for Secure Backup wrapping with device passcode, CoreFollowUp.framework; launchd service: com.apple.followupd
FoundationDB	fundamental iCloud storage database, marketed as CloudKit for app developers, separated into containers; records, blobs and large asset storage with MMCS, server-side continuous queries can trigger push notifications, user management by IDS, sharing between users; PCS keys used for hierarchical zone, record, and asset encryption; CloudKitDaemon.framework; launchd service: com.apple.cloudd; locally stored in ~/Library/Caches/CloudKit, ~/Library/Containers/*/Data/CloudKit
FUD	Firmware Update Daemon; /var/db/fud; launchd service: com.apple.MobileAccessoryUpdater
Gizmo	Apple Watch; managed by Bridge.app (watch settings) on Companion
GSS	Generic Security Service; part of Kerberos; GSS.framework; launchd service: com.apple.gssd, invoked by kernel through host special port 19; command line tool: gsstool
HAP	Home Automation Protocol; CoreHAP.framework
HDA	High Definition Audio; HDAInterface.framework
HDI	Hard Disk Image; command line tool: hdiutil
HDR	High Dynamic Range; video with wide-range transfer function; HDRProcessing.framework
HeadBoard	derivative of SpringBoard for tvOS home screen; sub-service: PineBoard
HLS	HTTP Live Streaming
HSA	Hardware Security Architecture; version 1 used for two-step verification, iCloud Keychain with iCSC; version 2 for two-factor authentication, SOS and Secure Backup with iCDP
HSM	Hardware Security Module; HSM fleet runs escrow service for Secure Backup; public keys for authenticating the HSM services in /System/Library/Security/Certificates.bundle/Contents/Resources/AppleESCertificates.plist
Hyperion	iCloud Photos, uses CloudKit; launchd service: com.apple.cloudphotod; command line tool: cpldiagnose
IAP	iPod Accessory Protocol; IAP.framework
iCDP	iCloud Data Protection, codename for a set of enhancements to iCloud privacy: device passcodes used as iCSC for Secure Backup, PCS root keys for CKKS-enabled services only synced between devices using SOS and not stored at Apple; launchd service: com.apple.cdpd
iCloud	umbrella term for a conglomerate of services, consists of FoundationDB containers with PCS views for key management, supported by SOS; uses IDS and APNS; some services under the iCloud name are actually served by the iTunes conglomerate or by IMAP or DAV
iCSC	iCloud Security Code, credential wrapping for Secure Backup, previously used a separate code, with HSA2/iCDP uses device passcodes
IDAM	Inter-Device Audio and MIDI; audio connection between devices
IDS	Identity Service, also IDMS, Apple ID identity management for all of Apple’s online services; APNS topics for signaling and messaging, see also Alloy, ESS, FaceTime, iMessage; authentication to services with Kerberos
IM	Instant Messaging; usually means iMessage and FaceTime, formerly also XMPP
Intent	use-case-driven interaction with 3rd-party apps from a host app; used for Siri, Maps, Widgets (configuration); extension points: com.apple.intents-service, com.apple.intents-ui-service
IOKit	device driver subsystem for in-kernel and DriverKit drivers, command line tool: ioreg
Ironwood	dictation, customized on server with selected user data (contacts, app names, music titles, HomeKit names, Siri Shortcut phrases), not tied to Apple ID; server: guzzoni.apple.com
ISP	Image Signal Processor; camera imaging circuit in iPhones
ITML	iTunes Markup Language; metdata tagging for media services; ITMLKit.framework
ITP	Intelligent Tracking Prevention, cross-site tracking defenses in Safari, statistics and user interaction classify sites, cookies are partitioned and access is restricted
iTunes	old umbrella term for a conglomerate of media services: App Stores, Apple Music, Apple TV, iCloud media library, Apple Podcasts, Podcast sync, Books Store, Books sync; server: phobos.apple.com
JARVIS	Just A Rather Very Intelligent Scheduler, Mesos cluster manager for Siri, iCloud, iTunes
Jellyfish	Animoji
Jetsam	reclaiming of purgeable memory and terminatable apps during memory pressure; see TAL
JSC	JavaScript Core; JavaScriptCore.framework; command line tool: jsc
Kerberos	single-sign-on mechanism; Heimdal.framework; command line tools: klist, ktutil
Kext	kernel extension, /Library/Extensions, /Library/StagedExtensions, /System/Library/Extensions; launchd service: com.apple.kextd, invoked by kernel through host special port 15; also handles DriverKit drivers; command line tool: kextutil, kmutil
Keybag	storage of protection class keys for Keychain and filesystem, protected by SEP with passcode and lockout; stored in user.kb; launchd services: com.apple.mobile.keybagd, com.apple.securityd_service, com.apple.secd
Keychain	storage for credentials; launchd service: com.apple.securityd; command line tools: certtool, security, systemkeychain
KIP	Kernel Integrity Protection, locking of physical memory pages to prevent changes to kernel
Launch Services	management for applications, uses Spotlight to update cached info; launchd services: com.apple.coreservices.launchservicesd, com.apple.lsd; CoreServices.framework/LaunchServices.framework; command line tools: lsappinfo, lsregister
Liverpool	PCS and TCC codename for CloudKit
LKDC	Local Key Distribution Center, Kerberos on client machines
LSM	Latent Semantic Mapping, text analysis, used for spam filtering, command line tool: lsm
Mac Buddy	historic name for Setup Assistant
Machine Learning	Vision.framework, Espresso.framework, Futhark.framework, PhotoAnalysis.framework
Madrid	iMessage; /System/Library/Messages
Manatee	PCS key for some CloudKit containers are protected/wrapped with a key synced via SOS, so data is unreadable to Apple (credential management codenames: Plesio, Stingray)
Mangrove	transfering UI tiles over XPC; Mangrove.framework, IOSurface.framework
Marco	Marco.framework, something about IDS and communication (iMessage, Calls), logging?
Marzipan	Catalyst; port of iOS frameworks to macOS, Catalyst apps are iOS apps with additional API to adapt macOS UI idioms; /System/iOSSupport; integration using UIKit system process; launchd service: com.apple.uikitsystemapp
MCX	Managed Client for OS X, preference management for settings from configuration profiles, /Library/Managed Preferences, command line tools: mcxquery, mcxrefresh
MDM	Mobile Device Management; server software to manage fleets of iOS and macOS devices; uses configuration profiles to manage preferences; ConfigurationProfiles.framework
MDS	Module Directory Services, ancient part of the old security APIs (CSDA, CSSM)
Memory Debugging	uses Taskport; command line tools: heap, leaks, malloc_history, stringdups, vmmap
Mesa	Touch ID; /Library/Catacomb; /var/db/bkad.db; command line tool: xartutil
Metadata	Spotlight; file indexing on macOS; CoreServices.framework/Metadata.framework, CoreServices.framework/SearchKit.framework; stored in /Volumes/*/.Spotlight-V100; launchd service: com.apple.metadata.mds; command line tools: mddiagnose, mdfind, mdimport, mdls, mdutil; in addition to auto-indexing, apps can explicitly register searchable items; CoreSpotlight.framework; launchd service: com.apple.corespotlightd
MMCS	MobileMe Chunk Storage, used by iCloud, splits blobs into chunks and stores them at Amazon/Google with convergent encryption; MMCS.framework
Mobile	prefix for iOS
Mobile Assets	demand-downloaded system components like fonts, dictionaries, linguistic data; stored in /System/Library/Assets/; launchd services: com.apple.languageassetd (language-dependent assets), com.apple.mobileassetd; server: mesu.apple.com
Mobile Device	connectivity to iOS devices over USB or WiFi (AirTrafficHost) for syning, development, and debugging; MobileDevice.framework; launchd service: com.apple.usbmuxd; Bonjour service: _apple-mobdev2._tcp
MOC	Managed Object Context; Core Data object space
Mondrian	photo arrangement in moments, collections, years; Mondrian.framework
MRT	Malware Removal Tool; /Library/Apple/System/Library/CoreServices/MRT.app
Multipeer Connectivity	ad-hoc networking; Bonjour for discovery; WiFi, AWDL, Bluetooth, or Ethernet as transport; optional encryption and certificate-based authentication
Nano	prefix for watchOS
Newton	fall detection on watchOS
NLP	Natural Language Processing; NLP.framework; related to mecabra libraries, a linguistic engine for Chinese and Japanese; /usr/share/mecabra, /usr/share/tokenizer
Notarization	app security scan by Apple; cryptographic proof stapled to code signature, tested by System Policy; command line tools: altool, stapler
Noticeboard	User Notifications for Software Update and App Store, Noticeboard.framework; launchd services: com.apple.noticeboard.state (nbstated), com.apple.noticeboard.agent (nbagent)
Notifications	system notification bus, unrelated to the local/remote push notifications; launchd service: com.apple.notifyd; command line tool: notifyutil; complemented by framework-level notification system (CFNotification, NSNotification); launchd services: com.apple.distnoted.xpc.daemon, com.apple.distnoted.xpc.agent
NSP	Network Service Proxy; per-app VPN and proxy settings; launchd service: com.apple.networkserviceproxy
ODR	On-Demand Resources; loaded from App Store; launchd service: com.apple.appstored
Onboarding	data protection splash screen shown by service-connected apps; /System/Library/OnBoardingBundles; OnBoardingKit.framework
Open Directory	directory service for user, group, and machine management; plugin-based to use different backend stores (LDAP, Active Directory), local accounts in /private/var/db/dslocal; launchd service: com.apple.opendirectoryd; command line tools: dscacheutil, dscl, dsconfigad, dsconfigldap, dseditgroup, dsenableroot, dserr, dsexport, dsimport, dsmemberutil, odutil
Opus	create slide shows from photos; Slideshows.framework
OSA	Open Scripting Architecture; scripting of applications from different fontend languages (currently AppleScript and JavaScript); backed by Apple Events; command line tools: osacompile, osadecompile, osalang, osascript, sdef, sdp
OTUT	One-Time Unlock Token; security mechanism to allow keybag unwrapping after updates
PAC	Pointer Authentication Codes; pointers signed in unused bits to prevent ROP attacks
Packages	unit of software installation; command line tools: pkgutil, installer, softwareupdate; launchd services: com.apple.softwareupdated, com.apple.bootinstalld, com.apple.installd, com.apple.system_installd, com.apple.uninstalld; /var/db/softwareupdate, /Library/Apple/System/Library/Receipts (system), /System/Library/Receipts (read-only), /private/var/db/receipts (App Store)
Parsec	Spotlight web results and searching of crowdsourced User Activity deep links; server: *.smoot.apple.com; launchd services: com.apple.parsecd, com.apple.parsec-fbf (Feedback Flush to Differential Privacy)
Pasteboard	storage for cut, copy, and paste; launchd service: com.apple.pboard; command line tools: pbcopy, pbpaste
PCS	Protected Cloud Storage; key management for separate iCloud storage compartments (PCS calls them views), each can contain FoundationDB plus bulk data stored by MMCS; see also iCDP, CKKS, Manatee; ProtectedCloudStorage.framework; /System/Library/Preferences/ProtectedCloudStorage
PCSC	Personal Computer Smart Card; PCSC.framework, uses CTK
PDE	Print Dialog Extension; old name, not a proper Extension
Pegasus	picture-in-picture video playback; Pegasus.framework (iOS), PIP.framework (macOS)
Pepper	UI elements for Watch home screen and Chat, like Quickboard (canned replies), Animoji; PepperUICore.framework
Persona	separation of sub-user-identities, like when using a private and managed Apple account; PersonaKit.framework; ~/Library/Personas; /System/Library/UserManagement
Piano Mover	Mail Drop; large mail attachments transfered over PCS; not to be confused with storage for iMessage attachments, which uses a CloudKit container
Plugin	Extensions, XPC services bundled with apps or frameworks, discovery by Launch Services; launchd service: com.apple.pluginkit.pkd; command line tool: pluginkit
PMP	Port Mapping Protocol; Apple alternative to UPnP, Bonjour service: _acp-sync._tcp
Preferences	storage for user-configurable settings; launchd services: com.apple.cfprefsd.xpc.daemon, com.apple.cfprefsd.xpc.agent; stored in Library/Preferences, command line tool: defaults; interaction with Synced Defaults per /System/Library/DefaultsConfigurations
Proactive	umbrella term for suggestions and completions based on Duet forecasting and User Activity context, also marketed as Siri features; PersonalizationPortrait.framework
QoS Classes	inheritable property for Activities; semantic priorities, influences scheduling parameters; initially set at user-level, priority inheritance within GCD queues and across XPC in kernel?
Quagga	framework for QR and barcode decoding; Quagga.framework
Quick Look	file preview and thumbnail generation; comand line tool: qlmanage
RAOP	Remote Audio Output Protocol; formerly AirTunes, now part of AirPlay; Bonjour service: _raop._tcp
Rapport	device pairing by proximity using Alloy, with PIN entry, or using iCloud; once paired, devices can access services; used for HomeKit, HomePod, AirPlay, Home Sharing, SideCar; Rapport.framework; launchd service: com.apple.rapportd; Bonjour service: _companion-link._tcp
Recents	recently used items (not files) in various applications, synced with Synced Defaults; CoreRecents.framework, /System/Library/Recents; launchd service: com.apple.recentsd
Relevance Engine	backend for Siri suggestions (for example of Siri Shortcuts), Siri watch face, Widget smart stacks; consumes Duet knowledge and app-provided timelines; /System/Library/RelevanceEngine; launchd service: com.apple.relevanced
Revisions	document autosave and auto-versioning; stored in /.DocumentRevisions-V100; GenerationalStorage.framework; launchd service: com.apple.revisiond
Routine	frequently visited locations on iOS, interacts with Duet; launchd service: com.apple.routined
RTC	Real-time Telemetry and Crash reporting; RTCReporting.framework; launchd service: com.apple.rtcreportingd
SBPL	Sandbox Profile Language; a TinyScheme-based embedded DSL for Seatbelt profiles
SCIP	System Coprocessor Integrity Protection; like KIP, but for SEP, ISP, Motion coprocessor
Screen Reader	VoiceOver and Braille; /System/Library/ScreenReader; ScreenReader.framework
Screen Time	digital wellbeing and parental controls system, uses Device Management as policy engine, self-enforced within the application by frameworks; launchd services: com.apple.ScreenTimeAgent, com.apple.dmd
SDB	SQL Database; CoreSDB.framework, used by iCloud communication
Search Party	portion of Find My service for offline devices; devices emit public part of rotating key pair via Bluetooth LE, other devices encrypt current location with this key and send to Apple, private key shared over CloudKit
Seatbelt	process sandbox by filtering system calls; /System/Library/Sandbox/Profiles, /usr/share/sandbox; profiles written in a SBPL; command line tool: sandbox-exec; launchd service: com.apple.sandboxd, invoked by kernel through host special port 14 for logging
Secure Backup	escrow part of SOS; escrow key individually wrapped with passcodes of trusted devices, stored in HSM to prevent brute forcing, uses SRP so passcodes are not visible to iCloud, limited number of recovery attempts; protocol called Lakitu, uses FollowUp; launchd service: com.apple.SecureBackupDaemon; CloudServices.framework
Secure Boot	firmware checks integrity of OS before booting it, configurable on Macs
SEP	Secure Enclave Processor; dedicated ARM core for security services, runs L4/Darbat, inline encryption to DRAM, factory-paired channels to Touch ID/Face ID hardware and Secure Element; SEP can use but not read device UID key, usage restricted to ROM code
Sharing	umbrella term for wireless proximity services: AirDrop, Continuity, Instant Hotspot, WiFi sharing; used by loginwindow for Watch unlock; Sharing.framework; launchd service: com.apple.sharingd; also serves connection sharing and remote disk
Shoebox	Passbook
Sidecar	using iPhone/iPad as Mac accessory: camera for photos and scanning, annotations, external display over low-latency WiFi (llw interface) using avconferenced encoding; SidecarCore.framework; launchd services: com.apple.sidecar-display-agent (SidecarDisplayAgent), com.apple.sidecar-relay (SidecarRelay)
Signpost	telemetry API to report points of interest in code; launchd service: com.apple.signpost.signpost_reporter
Simulator	running an iOS/tvOS/watchOS personality on macOS, uses sandboxing and a separate Mach bootstrap namespace for container-like isolation, command line tool: simctl
SKS	Secure Key Store; handling of keybag keys within the SEP
SkyLight	WindowServer; SkyLight.framework
Social Gaming	Game Center; multiplayer gaming services on top of CloudKit, shared storage and low-latency multicast for multiplayer sessions; launchd service: com.apple.gamed
Sock Puppet	Watch interaction that requires Companion device
SOS	Secure Object Sync; backend service for iCloud Keychain and iCDP/PCS, not to be confused with the emergency call feature; syncs credentials across trusted devices using keys from Circle, transferred items stored ephemerally in CKKS (new) or Synced Defaults (old) using OTR protocol; launchd services: com.apple.secd (access to local keychain), com.apple.security.cloudkeychainproxy3 (connects to Synced Defaults), com.apple.security.keychain-circle-notification; command line tool: otctl
SPI	System Private Interface; /System/Library/PrivateFrameworks
SpringBoard	iOS home screen; like Dock (Launchpad, Mission Control, desktop picture), Control Center, SystemUIServer (menu extras icons), loginwindow (lock screen), and WindowServer (compositor) on macOS; sub-services PreBoard, BaseBoard, FrontBoard, BackBoard (compositor), SplashBoard, SketchBoard; /System/Library/RunningBoard
SRP	Secure Remote Password; standard cryptographic protocol for proving knowledge of a secret such that attackers cannot brute-force the secret; AppleSRP.framework
SSO	Single Sign-On
Stark	CarPlay
Stockholm	Apple Pay
Storage Management	freeing up disk space by managing bulky items; UI in System Information.app; StorageManagement.framework; launchd service: com.apple.diskspaced; extension point: com.apple.storagemanagement; extends Cache Delete service
Suggestions	semantic analysis of mails and websites to suggest contacts, calendar events and the like; launchd services: com.apple.suggestd, com.apple.reversetemplated; custom JavaScript parsers in /System/Library/AssetsV2/com_apple_MobileAsset_CoreSuggestions
Symbols	debug symbols for backtraces; CoreSymbolication.framework; launchd services: com.apple.coresymbolicationd; command line tools: symbols, symbolscache
Symptoms	network diagnostics; Symptoms.framework; /var/networkd/netusage.sqlite; launchd service: com.apple.symptomsd, invoked by kernel through host special port 27
Synced Defaults	simple key-value store for applications, no user control over data; can use iCloud key-value backend (old) or Manatee container (new, marked as com.apple.kvs) as storage; launchd service: com.apple.syncdefaultsd; locally stored in ~/Library/SyncedPreferences
System Configuration	SystemConfiguration.framework; launchd service: com.apple.configd; command line tool: scutil
System Extension	user-level components formerly in the kernel; currently either a DriverKit, Network, or Endpoint Security extension; /System/DriverKit; launchd services: com.apple.sysextd, com.apple.nesessionmanager, com.apple.endpointsecurity.endpointsecurityd; command line tool: systemextensionsctl
System Policy	Gatekeeper; policy engine for application launches and kext loading; /var/db/SystemPolicy; launchd service: com.apple.security.syspolicy; invoked by kernel through host special port 29; command line tool: spctl
TAL	Transparent App Lifecycle; app process is started and stopped independently of the user launching and quitting app; also handles session restore across reboots; ~/Library/Saved Application State; launchd service: com.apple.talagent
Taskport	Mach kernel concept for ptrace-like access to task internals; access policy implemented by daemon; launchd service: com.apple.taskgated, invoked by kernel through task special port 9; command line tool: DevToolsSecurity
TCC	Transparency, Consent, and Control; user control over app access to privacy-related services (kTCCService*); TCC.framework; launchd services: com.apple.tccd, com.apple.tccd.system; command line tool: tccutil; stored in /Library/Application Support/com.apple.TCC, ~/Library/Application Support/com.apple.TCC, /var/db/locationd (for kTCCServiceLocation)
Time Machine	automatic backup service, command line tools: tmdiagnose, tmutil, bypass
Tin Can	Walkie Talkie on watchOS
Tones	ringtones; ToneLibrary.framework
Tourist	backend for user-visible tips and hints; launchd service: com.apple.touristd
Transparency	key transparency for ESS keys? Transparency.framework; launchd service: com.apple.transparencyd; server: init-kt.apple.com
TTS	Text To Speech, command line tool: say; /System/Library/Speech; synthesizer engines: MacinTalk (historic), Polyglot (phoneme-based?), Gryphon (current, DNN-based?)
TVML	TV Markup Language; declarative UI language for TV apps; TVMLKit.framework
Ubiquity	iCloud Drive; uses CloudKit, codename Bladerunner; CloudDocs.framework; command line tools: iclouddrivectl, fileproviderctl; launchd service: com.apple.bird (iclouddrive-agent); locally stored in ~/Library/Mobile Documents, ~/Library/CloudStorage/iCloud Drive
Unified Logging	system-wide logging and Activity tracking; launchd service: com.apple.logd, com.apple.diagnosticd; command line tool: log; /dev/oslog; data stored in /var/db/diagnostics, support files in /var/db/uuidtext
User Activity	abstraction behind deep-linking into apps with structured context data (people, places); used for Universal Links (with schema.org on websites), Handoff, Parsec, Siri Shortcuts, Proactive; UserActivity.framework; launchd service: com.apple.coreservices.useractivityd
User Notifications	user interface for notification center; launchd service: com.apple.usernoted
VA	Video Acceleration; AppleGVA.framework, AppleVA.framework, AppleVPA.framework
Viceroy	video conferencing used by FaceTime and ReplayKit
VSDB	volume status database; /var/db/volinfo.database; command line tool: vsdbutil
Waldo	VPN key management? location aware? seen in NSP, server: waldo.apple.com
Widgets	content excerpt from apps; provided via a timeline of views, configuration uses Intents
Willow	HomeKit; end-to-end-encrypted communication protocol and API for IoT-accessories; pairing with SRP using code printed on device, credential sync using SOS, transported over Alloy, remote access using Apple TV as proxy; launchd service: com.apple.homed
XCS	Xcode Server; continuous integration server; command line tools: xcscontrol, xcsdiagnose