IDFA

How it works

Each Apple device has an advertising identifier that apps can ask the system for. When two apps on the same device both read it, they see the same value, so an ad shown in one app and an install of another can be recognised as the same device.

Access is gated. In iOS 14.5, iPadOS 14.5 and tvOS 14.5 or later, an app needs the user's permission through the ATT framework to access the device's advertising identifier.^[1] Unless the user gives that permission, the identifier value is all zeros.^[1] A string of zeros is the same on every device, so it identifies nobody.

When it matters

It matters whenever you want user-level answers on iOS: which creative a specific paying user clicked, or how a user's later purchases trace back to one campaign. Those answers exist only for the users who allowed tracking.

It matters less than it used to for campaign totals. Apple's own frameworks count installs per campaign without the identifier, as described in what you can still measure without the IDFA.

How it relates to attribution

Comparing identifiers is the classic form of deterministic matching: the click record and the install record carry the same value, so the match is exact. The sister site explains this under deterministic versus probabilistic matching. On iOS today, an attribution tool uses the IDFA for the consented share of users, and reads SKAdNetwork or AdAttributionKit postbacks for campaign-level results.