Can someone explain how battle lines are drawn in static ops?

[noxnoctum]

So I'm playing a 4 battle operation, and doing setup for the 3rd right now. I've pushed deeper into the enemy's territory but would like to know how battles lines are drawn since I have one battle left. I have one giant red area (I'm soviet) where I can obviously put anyone anywhere and then a very small red area beyond my main line in the enemy's territory (since I had a lot of units there by the end of battle 2). But what's weird is right next to that small red area I see several blue bordered areas where I have units but cannot move them at all.

Can someone explain how the frontline is adjusted? How many units do I need in a certain area to have a forward setup zone?

[Sergei]

Like the shroud of Turin it's a mystery that cannot be explained, you just have to see and believe it. It also depends on the operation's "no man's land" setting and depends so much on where your forces are relative to enemy forces that explaining it would be futile. Think of it as a woman: sometimes she gives in to your advances, but sometimes you get thrown back when you thought you were doing so well. Que sera, sera.

[noxnoctum]

Hehe, I guess that answers that.

[Erwin]

If this is an Assault Op. units that are deemed to be "behind enemy lines" are fixed there and not returned to their own lines as in an Advance, neither will units "behind enemy lines" be supplied (according to the manual). If you saved the last turn of the previous battle it would be interesting to check what units received ammo resupply and which did not.