IFA is Identifier for Advertising which is an Apple Device ID released with iOS 6 and is a cross application / publisher identifier.
Unlike its predecessor, the IDFA is not permanently tied to a mobile device, as users can reset their IDFA at any time for added security or opt out of ad tracking altogether with a setting aptly named, “Limit Ad Tracking.” As a result, the IDFA can no longer be easily linked to specific devices/users.

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