not only units, also (most) objects
Say, ya put a trigger over the whole southern end of some island map.
And you & yer troopers are supposed to march towards the north..... then after you've
outbattled the enemy in the south & are now moving on to the north,
you can simply fire a trigger with THIS very simple spell :
"DeleteVehicle _x" Foreach ThisList
So, everything custom-added to the map will vanish!