DOCSIS modems require a configuration file from the cable modem termination system. This dictates the whole configuration from RF map and channel plan to things like QoS and management. Even with a customer-supplied modem, the CMTS will still supply a configuration file the modem must obey (and the CMTS will enforce).

For many modems on the customer market this also can mean that the ISP can push their own version of the firmware for a modem if you buy identical - such as pushing SURFboard updates.