Thanks, the Gamer's rulebook is on order awaiting the release of the Task Force rules. But I have the PDF, and if there haven't been many changes before publication is it indeed mentioned on p. 70, although sparsely.
Cytuj:
The enemy Unit is considered to be in the field of vision if we can see it from the perspective of our Unit [...] this must be at least a fragment of the Base of the enemy Unit in the case of vertical cover
Which I'm interpreting as you will have to measure from the heads of the shooter to the base of the target. This means that in some cases units on top of a hill will be able to shoot at units below, but they can't shoot back. I imagine a lot of heads will start spinning when this becomes a regular tactic.
Btw, the rule about agreeing beforehand what constitues cover only applies to
horizontal cover ("such as walls, fences, and ditches"). This seems to be a synonym for
obstacles ("include all kinds of fences, walls or narrow ditches").