In one line
ATT (App Tracking Transparency) is the iOS system prompt asking whether an app may track the user across other companies' apps and websites. Decline it and the advertising identifier (IDFA) is unavailable.
Why the numbers move
Without IDFA, user-level attribution breaks. Those installs are visible only through aggregate frames like SKAN, and the counts in your ads manager, MMP, and payment database start to diverge.
The common mistake follows: iOS looks cut in half, so the campaign gets paused — when what actually shrank was measurement, not performance.
Opt-in rate is manageable
When and in what context the prompt appears changes consent substantially; priming — showing value first, then asking — is the standard approach. Raise consent and the measurable share of your sample grows, and iOS performance starts to reappear.
Go deeper
What to judge iOS performance by after ATT is covered in the iOS ATT and SKAN measurement guide.