Apple's App Tracking Transparency framework, introduced with iOS 14.5, lets iPhone users opt out of being tracked across apps. Meta introduced Aggregated Event Measurement in response, processing conversion data from those users in an aggregated, delayed form and limiting how much individual-level detail is reported.
AEM originally required advertisers to verify their domain and rank a limited number of priority events. Meta has since simplified much of that configuration, but the underlying effect remains: some conversions from iOS users are modeled or reported with delays, so Meta's results should be read alongside a CRM or analytics source of truth.
Example
An advertiser notices that conversions from iPhone users appear in Ads Manager a day or two after they happen and are partly modeled — the effect of Aggregated Event Measurement.
How Sloss Digital uses it
We verify domains, configure events correctly, and reconcile Meta's reported results against CRM data, since iOS privacy rules mean the platform can't see everything.