[jayrhynas]

There are two identifiers: Identifier for Advertisers (IDFA) and Identifier for Vendors (IDFV).

IDFA is the same across all apps on a device. However, it can be reset by the user or disabled (in which case it returns all 0s). Also, apps have to disclose (to Apple) that they use the IDFA - not sure if that's visible to the user in the App Store anywhere.

IDFV is unique per vendor - that is, each app has a different ID, but two apps from the same developer will have the same ID. I believe this is also reset when resetting the device.

The FBSDK doesn't require developers to enable the IDFA, so the unique identifier in the phone home request is either the IDFV (effectively unique) or just a UUID that the FBSDK generates and stores on launch.