asterisk users - Jan 2007 - SIP invite and sip.conf relationship?

I'm having a bit of trouble setting up my sip.conf entries to accept
calls from a particular provider, and the problem really is that I am
unclear exactly what parts of the INVITE are supposed to match what
parts of sip.conf.

I couldn't find this info on the wiki, so if someone here can shed
some light, I would be very grateful!

Here are the relevant lines from the INVITE (from sip debug):

<-- SIP read from 213.166.5.130:5060: 
INVITE sip:username@sip.mydomain.com SIP/2.0
From: "01234567890"
<sip:01234567890@123.123.123.123>;tag=2F6B6198-D3D
To: <sip:448451234567@n.e164.org.uk>
Date: Wed, 10 Jan 2007 18:18:22 gmt
CSeq: 101 INVITE
Remote-Party-ID:
<sip:01234567890@123.123.123.124>;party=calling;screen=yes;privacy=off
Contact: <sip:01234567890@123.123.123.124:5060>

How do the items above, such as source address, INVITE URL, From, To, etc.,
relate to items in sip.conf in a type=user section, such as [sectionname],
user=username, host=hostname or host=dynamic, etc?

My provider gives me the option to set the invite URL, such as
sip:sip.mydomain.com or sip:myuser@sip.mydomain.com, but does not use
a secret to authenticate. Does the myuser part get used at all?

Thanks for any insight.

Cheers
Tony
-- 
Tony Mountifield
Work: tony@softins.co.uk - http://www.softins.co.uk
Play: tony@mountifield.org - http://tony.mountifield.org