App Tracking Transparency (ATT) framework
As per Apple's guidelines, any app that collects end-user data and shares it with third parties for tracking or advertising-related purposes must use the AppTrackingTransparency (ATT) framework. This requires app publishers to obtain explicit consent from users to access their iOS advertising ID (IDFA).
To gain user consent, the ATT framework presents an app-tracking authorization request to the user. A user can still control privacy at the app level or block all apps from tracking them at a global level.
By default, a user is opted-out of tracking when the app launches. To gain user consent, the ATT framework presents an app-tracking authorization request to the user. The ATT request works on a per app basis, meaning that users can decide to opt-in for specific apps as they choose.
For users who opt-out of tracking, the IDFA returns a string of 0s. For users who opt-in, the IDFA is available and can be used for attribution purposes.
A user can choose to control privacy at the app level or even set Allow Apps to Request to Track to Off to block all apps from tracking them at a global level. The ATT consent pop-up is displayed only if the user has this setting On in the device settings.
If Allow Apps to Request to Track is changed during the app’s lifetime, this is what happens: