Oh jeeze, sorry for the delay!
An active theater of war, as listed in the module, is pretty much the Bridge/No-Man's-Land, and hostile territory where the enemy army is presently fighting. It is meant to shift but I likely didn't convey that as well as I could in this initial draft.
A better version of the Battlefield, with rulings to make your own battlefield and such will be in Rakehell 2 which I hope to have out in a month or so.
I'll get back to you with a better read on this in an hour or so, as I just noticed this while in the middle of something.
- Brian