ATT
Also: App Tracking Transparency · IDFA prompt
ATT (App Tracking Transparency) is the iOS permission prompt, introduced in iOS 14.5, that apps must show before accessing the device advertising identifier (IDFA) for cross-app tracking. Users who decline cannot be attributed at the user level.
Opt-in rates are typically well under half of users, so deterministic, IDFA-based attribution covers a minority of iOS installs. Everything else runs on SKAdNetwork and AdAttributionKit, or on probabilistic methods that Apple's policies restrict.
ATT is why iOS measurement is aggregate by default and why an MMP's iOS reports are a blend of deterministic, SKAN, and modeled data. Check which is which before trusting a number.
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