While not the rule, generally OPT is uded for DMZ. I beleive OPT is
just short for optional interface. With m0n0wall (as well as other
smiliar applications), you can have several interfaces behind the WAN.

The point of the DMZ is to run your public servers on a different
network. Different from say your business network(LAN). There have
been several tha have posted and additional OPT interface for a
separate wireless network.

If you want an excellent book on firewalls. check out "building
interenet firewalls", ISBN: 1565928717, Amazon has them new and used
starting at $4.20 US.

http://www.amazon.com/gp/product/1565928717/104-0891883-3173535?v=glance&n=283155

~ Don