It's a new gateway device that BellSouth is deploying in some areas (it's on the telco side, not on your end). It will allow them to provide access service to multiple ISP's over common equipment (instead of each ISP having to deploy their own access hardware). The domain name must be appended to the username so that the BBG will know where to route the authentication request. If your line is not served by a BBG, the domain name is not required for authentication.