By default, apps are created with user segments that are configured to identify users based on app-specific user IDs, also known as the Player Identity Lookup. It is the recommended lookup option since it allows TROPHiT to manage per-user redemptions based on their actual user ID within the app.
However, when such user IDs are not available for any reason, TROPHiT can be configured to identify such users based on device IDs, also known as the Device Identity Lookup. In such cases, every redemption would be associated to the appropriate device ID: IDFA on iOS or GAID (falling back to ANDROID_ID) on Android. While not necessarily in sync with app-specific user IDs, this method may be as reliable while users do not reset their device's advertising ID.
You can change your user segments' identity lookup configuration under each segment's Settings page.