Tracking
ATT (App Tracking Transparency)
Apple's iOS 14.5+ opt-in prompt for cross-app tracking.
ATT (App Tracking Transparency, iOS 14.5, April 2021) is Apple's opt-in prompt that requires apps to ask users for permission before tracking them across apps and websites. Opt-in rate: ~25% on average, with high variance by category (gaming 18%, news 35%, social 22%). Implementation: apps must call AppTrackingTransparency framework. Net effect: roughly halved the IDFA-based attribution accuracy in mobile UA.
Example
Mobile gaming UA campaign on Applovin sees CPA 'rise' from $4.50 to $6.80 after iOS 14.5 — actual install volume unchanged; the gap is signal-loss from ATT-opted-out users no longer attributable to the install source.