Google’s Advertising ID (GAID)
Like the Google Play Referrer, Google Advertising ID is an extremely accurate identifier. This unique and anonymous device ID is used, among other things, to measure Android installs (for both Google Play and Android out of store). The main issue however is that it’s not always available: to be passed in a click or view, Google Advertising ID requires configuration on the ad network side; it can also be disabled by the user to prevent any profiling and the delivery of personalized ads, although very few actually change this default setting; lastly, it does not work on the mobile web.
IDFA is Apple’s highly accurate and privacy-sensitive device identifier. This method is similar to Google Advertising ID in that it provides a unique identifier for a device/user to attribute, or track, installs through the App Store. IDFA has two key features: It can be reset by the user at any time and users can opt out of profiling and personalized ads by enabling a device setting called, “Limit Ad Tracking.” IDFA replaced UDID which could not be changed by either the user or Apple.