There is a limit to 100 devices that you can register. Each registered device takes 1 "place" out of 100 that you have. You can return this space only once a year - when updating your account. When you upgrade your account, you again have the choice to turn on devices.
But after registering the device, you cannot unregister before the update process. You can disable it, but it will still take 1 "place" out of your 100 devices.
Edit: if you disconnect the device, the provisioning profiles containing this device will be invalid, so just edit them and create them again
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