| Apologies, I'm conflating two slightly different things there. There's the identifierForVendor [0] which is unique to the publisher. This is pretty safe to use however you see fit (within reason). Then there's the advertisingIdentifier [1], which is not unique, but can easily be permanently zeroed out by the user. Apple also have some fairly stringent rules about how it can be used [2], not to mention further rules about not identifying people surreptitiously [3]: > 5.1.2 Data Use and Sharing > (iii) Apps should not attempt to surreptitiously build a user profile based on collected data and may not attempt, facilitate, or encourage others to identify anonymous users or reconstruct user profiles based on data collected from Apple-provided APIs or any data that you say has been collected in an “anonymized,” “aggregated,” or otherwise non-identifiable way. They ask you to explicitly confirm that you're following the advertising identifier rules in particular every single time you submit to the App Store. [0] https://developer.apple.com/documentation/uikit/uidevice/162... [1] https://developer.apple.com/documentation/adsupport/asidenti... [2] https://support.appsflyer.com/hc/en-us/articles/207032086-Ap... [3] https://developer.apple.com/app-store/review/guidelines/#dat... |