[Serusers] How many expresions in contact?

[edison]

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 at domain.com
	contact: sip:john.doe at domain.com:5060
	contact: sip:john.doe at domain.com expires=3600
	contact: sip:john.doe at 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.