The device list under My Account shows devices for which you have previously purchased a feature licenses or obstacle database, as well as devices that you have added manually. These devices are used to automatically populate the drop-down list when e.g. purchasing an obstacle database.
If you are no longer using a specific device, it is advisable to delete the device from your device list, so you don't accidentally purchase an obstacle database for the wrong device. You can do this by clicking the 'Delete' link below the Manufacturer name in the list.