Usually they will need at least the MAC address (different manufacturers say it differently so it might be MAC Id or HFC/Cable MAC), they will also need the serial number. I usually look at the S/N not the circled Customer S/N one - because the customer one is not reported by the modem on the CMTS side, just the rather long one that isn't circled. Please note: picture isn't mine. If you can't read the label there are two tricks: http://192.168.100.1/cmAddress.htm gives you the proper serial number and HFC MAC Address; or take a picture of the label with your smartphone and blow up (zoom in) the picture to make it easier to read.
Hi DougSchuess, What grymwulf says is correct to register a new modem we would need the model of the modem the CM/HFC Mac address and the customer serial number to register it with the service and activate it.