App Activity Custom Audiences
App Activity Audiences, one of three types of Custom Audiences, are built from mobile app events that are provided from a mobile tracking partner. People who have taken a specific action in your app, such as install or sign up, are then added to the Custom Audience which you can re-target in campaigns. To set up App Activity Audiences from mobile apps, you must be working with a mobile app conversion tracking partner.
This precise targeting option is a great way to leverage existing app users and drive in-app conversions.