Data & identity · also called Identifier for Advertisers
IDFA
IDFA (Identifier for Advertisers) is Apple's resettable advertising identifier for iPhones and iPads, which apps can access only if the user grants permission through App Tracking Transparency.
Apple introduced the IDFA in 2012 as a privacy-friendlier replacement for permanent hardware IDs. It is a random identifier that apps can read to recognise a device for advertising, and that users can reset.
For years it was on by default, and it underpinned iOS ad targeting, retargeting and install attribution. That changed with iOS 14.5 in April 2021: under App Tracking Transparency, apps must show a permission prompt before reading the IDFA or otherwise tracking users across other companies' apps and websites. If the user declines, the app receives a string of zeros. Opt-in rates vary widely by app and market, and are commonly far below half of users.
Without the IDFA, iOS advertisers rely on Apple's privacy-preserving attribution, SKAdNetwork and its successor AdAttributionKit, on first-party data and contextual signals, and on aggregated modelling. Apple's rules also forbid using device fingerprinting as a substitute. The Android equivalent is the GAID.
Think of it like this
The IDFA is like a visitor badge an iPhone can wear for advertisers, but since 2021 it only wears it if its owner says yes.
An example
A mobile game shows the ATT prompt to 1 million US iOS users; 28% allow tracking and share their IDFA, while the other 72% are measured through AdAttributionKit and aggregated reporting.
Related terms
App Tracking Transparency (ATT)
App Tracking Transparency (ATT) is Apple's framework, introduced with iOS 14.5 in 2021, requiring apps to ask users' permission before tracking them across other companies' apps and websites or accessing the IDFA.
Mobile advertising ID (MAID)
A mobile advertising ID (MAID) is a resettable identifier that a phone's operating system provides for advertising, such as Apple's IDFA or Google's GAID, letting apps recognise the device for ads.
GAID (Android advertising ID)
GAID, the Android advertising ID, is Google's user-resettable identifier provided by Google Play services that Android apps can use for advertising and analytics.
SKAdNetwork / AdAttributionKit
SKAdNetwork and its successor AdAttributionKit are Apple's privacy-preserving frameworks that tell advertisers which ad network drove an iOS app install, without revealing user-level identity.
Signal loss
Signal loss is the reduction in data available to target, measure and optimise advertising, caused by browser cookie blocking, mobile ID restrictions, privacy laws, consent choices and ad blocking.
Sources: Apple Developer: advertisingIdentifier, Apple Developer: App Tracking Transparency