Identifier for Advertisers (IDFA)

IDFA (Identifier for Advertisers) is a UUID Apple assigns to each iOS device for ad-tracking purposes. Before iOS 14.5 (April 2021), it was universally available to any installed app — MMPs read it on click and on install, matched the two, and attributed installs deterministically. This single mechanism underpinned mobile-app attribution from IDFA's introduction in 2012 until ATTGlossaryApp Tracking Transparency (ATT)Apple's iOS 14.5+ framework requiring apps to display a system prompt and obtain user consent before accessing the IDFA or tracking the user across other apps and we... (App Tracking Transparency) rolled out.

What ATT changed: iOS 14.5+ requires apps to display a system prompt and obtain explicit user consent before accessing the IDFA or tracking the user across other apps and websites. The prompt is rendered by iOS — Apple controls the wording, no dark patterns. If the user denies, the IDFA is returned as all zeros (effectively unavailable) and the app cannot track the user across other contexts.

Opt-in rate variability by app category

• Consumer apps with clear value-exchange messaging (streaming, dating, finance with personalization promised): 40-50%.
• Productivity and utility apps: 25-40%.
• Casual gamesGlossaryCasual GamesA broad genre of easy-to-learn, short-session mobile games — the highest-volume segment of mobile gaming, monetized through a hybrid of ads and in-app purchases and...: 20-30%.
• Hyper-casual games: 15-25% — users have lowest investment, least patient with prompts.
• Heavily-branded consumer apps: 50%+ — users trust the brand enough to grant consent.

Apps that prep users for the prompt (a pre-screen explaining what tracking enables) usually lift opt-in 10-20 percentage points vs cold prompts.

Attribution stack post-ATT

1. 01For the 20-40% who opt in: classic IDFA-based deterministic attribution still works.
2. 02For the 60-80% who opt out: SKAdNetwork (SKAN)GlossarySKAdNetwork (SKAN)Apple's privacy-preserving attribution framework for iOS — returns install credit to ad networks with a deliberate delay and aggregation, without exposing any device... handles attribution at the aggregated, fuzzed, delayed level.
3. 03Probabilistic / fingerprintingGlossaryFingerprinting (Probabilistic Attribution)Probabilistic identification of devices based on combined signals (IP, device model, OS version, screen size) rather than deterministic identifiers — explicitly rest... attribution: was widely used in 2021-2023 as a fallback; Apple's restrictions on fingerprinting tightened over time, making it less reliable now.
4. 04First-party signal (CRM, web-to-appGlossaryWeb-to-AppA user acquisition flow where users discover and engage with your product via web first, then are converted to install the mobile app — often with deferred deep link..., email-to-install): grows in importance as third-party tracking fades.

Android: GAID (Google Advertising ID)GlossaryGAID (Google Advertising ID)Google's per-device advertising identifier on Android — the equivalent of Apple's IDFA. Still broadly available in 2026, but Privacy Sandbox for Android will eventua.... The equivalent identifier on Android, still broadly available as of 2026. Users can reset it or opt out (Settings → Google → Ads), but the opt-out rate is much lower than iOS ATT — typically 10-20% on Android vs 60-80% on iOS. Google's Privacy SandboxGlossaryPrivacy Sandbox (Android)Google's multi-year initiative to introduce privacy-preserving alternatives to GAID-based ad tracking on Android — the platform's eventual SKAN equivalent. for Android (in phased rollout) will introduce SKAN-like aggregated attribution by ~2027, gradually restricting GAID. Until then, Android attribution remains primarily deterministic.