In the Secured Area battle builder condition, each side gets 50 AP of fortifications to place, limited to Minefields, Dug-In, and Camouflage. However, there is nothing about where these can and cannot be placed. Is there any official ruling on how this is supposed to work?

It can make a huge difference. If limited to your deployment zone, they can be worthless and/or overcosted depending on the objective/situation ("Men, we mined our own position"). If unrestricted, you can drop a Dug-In unit or 5 into your opponent's deployment zone (surprise!). Some official guidance would be greatly appreciated and this might be a great candidate for a future FAQ/errata.

Mike J said:

Dug-In and Camouflage are markers that are placed on a unit that you have deployed. You can't randomly deploy a unit outside your deployment zone.

Mines, I would need to re-read to be sure, but I believe they are placed just like any other piece of terrain.

Agreed on Dug-In.

A Camouflage marker is placed "in an area of terrain" instead of deploying the unit.  If it were a marker placed on an already deployed unit, there would be no question.

The only part of Minefields that I can find that comes even close to how to place them is "A minefield is designated by a marker placed face down on the tabletop."  I couldn't find anything that says to treat them like terrain.

I'm not trying to argue these one way or the other.  I'm just looking for an official answer (existing or otherwise) that I can point to when another argument over this comes up.