There are several different types and names of device IDs. For example, on iOS the device ID is referred to as the Identifier For Advertisers (IDFA) and is composed of 8 digits followed by a dash and three sets of 4 digits. As mentioned, with the introduction of iOS 14.5 and App Tracking Transparency, users can opt out of sharing their device ID with apps and advertisers. After an app is installed the user is prompted with an opt-in or opt-out prompt on an app-by-app basis.
The device ID on Android is referred to both as the Android Advertising ID (AAID), and the Google Ad ID (GAID), which can be accessed from the Settings menu. Here users can opt-out of sharing their device ID with advertisers, which means they will not receive any ad personalization.
Users that have opted out of sharing this data are referred to as ‘Limited Ad Tracking’ users.
The main use case for advertising IDs is for advertisers to analyze and understand user behavior in order to show the right ads to the right people, and to and identify optimization opportunities. With this data, advertisers typically use a marketing analytics platform to improve the performance of their campaigns and increase the ROI of their marketing efforts. For example, with a device ID advertisers can track which users have seen their ads, which ad creatives they have seen, and whether or not they converted into an app install.