Your Devices List should always be an accurate reflection of the active devices you are using in your store. This is particularly important for Register Devices because adding unused registers could result in additional fees.
There are also times where you may need to Clear Cache off of your Register by deleting the Register App. If you delete and don't circle back to remove the previous device from your Devices List, this can quickly lead to an outdated list of Registers.
You should remove any unused devices to keep this list clean. From your Devices List > click on the Device Name > Remove Device.
Any Devices that are unlinked will be missing a UUID number. To see if a UUID number is missing, from your Devices List > click on the Device Name > Edit Device. If there is a blank space next to the lock icon, there is no linked iPad or Device for this Register.