Mobile advertising IDs allow mobile app developers to identify who is using their mobile apps.

There are three types of mobile advertiser IDs that we support for Custom Audiences:

• Apple's Advertising Identifier (IDFA): An advertising ID that Apple provides as part of iOS in its ads framework
• Android's Advertising ID: An advertising ID that Google provides as part of Android
• Facebook App User IDs: An ID corresponding to someone who uses an app that can be retrieved through the Facebook SDK.

Mobile advertising IDs may be useful with Custom Audiences if:

• You want to reach people already using your app with Facebook ads on desktop or mobile
• You want to reach people who have taken a specific action in your app (such as a purchase) with a specific message
• Your app doesn't use a Facebook login, but you want to reach people who are already engaged with your app

* Nguồn: Facebook