asterisk users - Sep 2006 - Difference between "SIP Server" and "Outbound Proxy"

It seems a number of phones have separate settings for "SIP Server"
and
"Outbound proxy".

More specifically, the Linux X-Lite softphone I've been playing with has
"SIP Proxy" and "Out Bound Proxy" settings, and it seems the
GrandStream
BudgeTone 100 has "SIP Server" and "Outbound Proxy"; see
http://siproxd.sourceforge.net/siproxd_guide/siproxd_guide_c6s2.html

Now, my question is: what exactly is the distinction? Reading RFC 3261, the
concept of an "outbound proxy" is clearly defined - all outbound SIP
packets
will have their destination set to this address. I'm fine with that. But
what's the separate "SIP Proxy" or "SIP Server" setting
for?

By a process of experimentation on X-Lite, I have determined:

* All outbound requests are sent to the "outbound proxy", unless I
have
  set "use outbound proxy: never", in which case they're sent to
the
  "SIP proxy"

* I don't see any other circumstance in which the "SIP proxy"
setting is
  used. I've done some tcpdumping of SIP traffic, and may have missed
  something. However if I point the "SIP server" setting to some other
  host in my network, I don't see its address being used at all - neither
  registration nor call origination.

More specifically: if I set the "Outbound proxy" to 10.69.255.251 (an
Asterisk server), and the "SIP proxy" to 10.69.255.252 (another PC),
then
all REGISTER and INVITE requests are sent to 10.69.255.251. No packets are
sent to 10.69.255.252, and as far as I can tell, none of the packets to
10.69.255.251 include 10.69.255.252 in their contents either.

So I don't understand what the other "SIP Proxy" or "SIP
Server" setting
is for, and I couldn't find that info on voip-info.org or google.

If anyone could explain, clearly and precisely, how this setting is used by
the client (in terms of how it affects the SIP exchanges), I'd be very
grateful!

Many thanks,

Brian Candler.