[Serusers] How many expresions in contact?

Hi: I am working on softswitches and majored in SIP protocol.During my works I found something confused me. In [contact] head field there are many expresions to see , and seems they are all right.So I searched multi documents to find out how many expresions are present. For example: contact: sip:john.doe@domain.com contact: sip:john.doe@domain.com:5060 contact: sip:john.doe@domain.com expires=3600 contact: sip:john.doe@127.0.0.1 And so on. I am really confused and wondered how many things can 'contact' do,how many informations can 'contact' provide? Anyone who could aguide me where I can make sure what contact do? Regards edison.