Thanks, but I originally wanted to entrench some AAA units. Well at least someone replied
(Also I didn't know about that foxhole you've mentioned. So thanks anyway)
You cannot entrench only a few selected units. In an assault/defend mission, the defender is dug-in, all of them, unless deployed in a marsh or a few other places where trenches are not allowed. The assaulter will have mortars, and infantry that deploy on foot etc in trenches.