They're only found in Cmak and have to be placed by the scenario designer when he's creating the scen, they can't be bought in QB's.
As cover goes, they rate 30% which is about 'average', but they give NO cover if a firing enemy is appreciably higher.
A foxhole in an emplacement gives no extra cover.
Tanks get no cover in Emps, not even hulldown.
Only full-tracked vehs and foot units can cross or enter them, but guns can start the game in there.