What is the 'domain' in authentication

I want to send out a SOAP request with username/passoword. I have filled in the username and password under the 'Auth' tab. The request if failing (ter:NotAuthroised'). When I check with wirehsark, the authentication elements of the soap bodyt are not being included. I have left the 'domain' field blank and was wondering if this could be causing the problem. What do I fill in for 'domain'? (the 'Outgoing WSS' and 'Incomming WSS' fields are also empty).