A key is generated from each device and assigned to it as its unique device ID. Once a user returns to the app, Insider can detect which user is back on the app. This ID is assigned to the device's motherboard, and the user cannot change it.
Each time the app is deleted, a new UDID is assigned to the device. However, if different users use an application on the same device, their UDID would be the same.
You can use UDID to segment devices and detect your device in the Test Lab when testing your events and attributes.
