The service keeps track of devices used by the users of your app (mobile or web) and allows you to limit the usage by each user to a small number of devices (default 5). This limitation can help reduce credential sharing on paid services.
Mobile devices should be uniquely identified by the deviceID that is also used for push notifications.
Web applications should use the userAgent field of the client. This is technically not a unique identifier of the device, but can be used as a close approximation.