The UID is a 16 digit hexadecimal identification number that is unique to your Untangle NGFW software installation. It is not tied to your hardware and will be regenerated if you reinstall the software. You can find the UID by going to Config > About.
You can also find your UID in Command Center by going to the Appliances tab at the top of the screen. If your UID is obfuscated (shows a lot of Xs, like in the image below), click here to fix that.