Registrar/Proxy - is either the IP Address or domain of the SIP server that you want to register with.

SIP Domain - is the domain name to be suffixed to the account “user” entry when creating that REGISTER request and specifies the SIP URI the client wishes to be REGISERed to.

EXAMPLE

If "User" is "100" and "SIP Domain" is "sip.clearcom.com", then "100@sip.clearcom.com" will be REGISTERed with the SIP server "sipserver.hme.com".

If "User" is "100" and "SIP Domain" is "sip.hme.com", then "100@sip.hme.com" will be REGISTERed with the SIP server "sipserver.hme.com".

See also What does the Domain field setting in the LQ SIP mean?