On mobile apps we use a specific identifier sent by the SDK to define Unique Visitors. If a user does not wish to be tracked, they can opt-out.
Depending on the OS, several IDs can be used:
- Android, you can decide whether you want to use the Android ID (ID given by the phone, will remain the same even if the app is deleted and reinstalled) or the Advertising ID (provided by Google, for an Android device. It is unique, but can be reset by the user. It is also subject to the user's choice to disable ad tracking, in which case the user cannot be identified).
Find out more in the online help (Android (Java)).
- Apple, you can either use the IDFV (the ID will be shared across all the apps of the same publisher), or the IDFA (the ID is unique to the phone)
Find out more in the online help (iOS (Swift)).
Like on browsers, you can also define your own ID, called the UUID.