Rule question

[Patrick Petermair <captain.nuke@gmx.at>]

Hi!

I've read the following example script for a linux box with masquerading and 
some firewall rules:
http://www.ecst.csuchico.edu/~dranch/LINUX/ipmasq/examples/rc.firewall-2.4-stronger

So far I think I understand everything except this line:
$IPTABLES -A OUTPUT -o $INTIF -s $EXTIP -d $INTNET -j ACCEPT

How can an OUTPUT of the internal interface have the external IP address as 
source IP ?? And why would it be accepted?
As far as I understand it, the masquerading and stuff is all done in the 
FORWARD chain. I can't think of an example where a packet originating from 
the internal interface of my firewall can have the external IP as source.

Regards
Patrick