These are devices that are hard linked to a user's account. It was in use in old versions when it was needed.
You can switch the Disconnect/Connect toggle to deactivate a linked device, but it is a potentially harmful option, so your users will not be able to log in using this device.
Devices in Use
These are devices that logged in to the user's account, it may be STB/WebTV/Phone/Tablet and other devices. The table contains the following columns: