The ATTBI network supports both DOCSIS and non-DOCSIS cable modems (CMs). DOCSIS CMs are available both from AT&T and from retailers. Non-DOCSIS CMs are generally only available though AT&T for purchase or lease (most customer's have leased). If you bought your CM recently (e.g. within the last year) from a retailer it almost certainly is a DOCSIS modem. If you lease your modem, it may or may not be DOCSIS.
This is important because CM registration is only required for DOCSIS CMs. Non-DOCSIS users don't need to register their modems. Registering a non-DOCSIS modem will have no effect on your service; generally it will produce an error message from SAS.