Mobile advertising ID (MAID)

Web browsers have cookies; mobile apps have advertising IDs. Apple's is the IDFA and Google's Android version is the GAID. Each is a random string the operating system assigns to a device, shared with apps that request it, and that the user can reset or disable.

MAIDs power in-app advertising: targeting, frequency capping, retargeting, and above all app install attribution by MMPs. Because the same ID is visible in every app on the device, they also allowed extensive cross-app tracking and a large trade in location and app-usage data, which has attracted regulator attention.

Since 2021, Apple's App Tracking Transparency requires apps to ask permission before accessing the IDFA, and many users decline, sharply reducing MAID availability on iOS. Android lets users delete the ID, and apps then receive a string of zeros. Fraudsters also abuse resettable IDs, making one phone look like thousands of new devices in device farm schemes. On CTV, a similar resettable ID exists on many TV platforms.

Think of it like this

A MAID is like a name badge your phone wears when meeting advertisers; you can swap it for a new one, or, on iPhone, refuse to wear it at all.

An example

A shopping app in Brazil sees GAIDs on 90% of its Android users but IDFAs on only about a quarter of its iOS users after ATT prompts, so its iOS campaigns rely more on SKAdNetwork and modelled measurement.