Unknown argument in UnitDamageTarget / UnitDamagePoint

What does the argument "boolean attack" do?
I've tried using both true and false it still does the same.

I suspect it decides if WC3 treats it as an attack (and thus open to stuff like hardened skin) or not. But it's quite possible it does nothing.

It could probably trigger the "A unit is attacked", but I doubt it. I think that'd be something worth a quick test. (Use a trigger to inflict X damage on two units; one unit has hardened skin, the other doesn't.)

trigger damage run no event expect damage.