What is ATT?

Before ATT, an app like Facebook could read Apple's advertising identifier (IDFA) by default and stitch a clean person-level trail from ad impression to off-app purchase. ATT made that tracking opt-in, and reported opt-in rates have generally sat in the low tens of percent, which means most iPhone conversions can no longer be tied to a specific user with certainty. The practical fallout for ecom advertisers: smaller and slower-firing retargeting and lookalike audiences, conversion windows that report fewer events, and reported ROAS that drifts from what backend revenue actually shows. The industry adapted with Aggregated Event Measurement, modeled conversions, and the Conversions API (CAPI), which sends purchase data server-to-server so the platform can still learn even without the device identifier. It is genuinely a two-sided story. For users it is a real privacy gain, since cross-app tracking now requires consent. For advertisers it traded precision for privacy: the data is fuzzier and more statistical, but aggregate optimization and CAPI mean campaigns still work, especially when paired with strong creative and a clean post-click funnel. ATT applies to iOS only; Android still uses Google's advertising ID under a separate, more advertiser-friendly regime, though Google's own Privacy Sandbox is moving in a similar direction.