I think it has something like 4 options. Press Once: Lower. Press Twice Higher, Press 3 Times Even Higher and 4x for Reset.
If you go for a POV rather than a over top view, you will see it shift height.
It is also cumulative meaning it does not reset. Hit shift.. switch player and its shift based off of what it you left it. SO hit one time shift (lower).. switch player hit shift one time( now higher) switch to a 3rd person and hit shift (even higher). Doesn't reset when you shoot, either adjust it every aim or just reset it.
It also relates to your targeting reticule, or without clicking on a target your mouse will level itself to the terrain. You can adjust the arc or projectile line by aiming beyond/before a target but the terrain height will alter/change the vertical levels just by itself.
So with a straight projectile, you want a green to red line, red being when it hits on a true path. If you aim beyond the target you can aim for the sides rather than true mass. ie instead of shooting through a post you may be able to just get around it.. or Guy on the right you don't want to hit.. aim beyond the target and a lil left.. ( so long as the line goes red at the target.) So if you miss left.. you miss by a lot but if you miss right.. hopefully you hit the ALiens and not by enough to hit your own soldier.