Hi, thanks for the report!
It’s terribly named and due me getting it switched around early on and now hard to change without breaking the stuff built on it, but the “unit” input is where you put the unit getting damaged, and the optional “target unit” here is the character that dealt the damage:
I’ll update the effect description to make that more clear. This also means that it doesn’t provide an easy way to damage multiple units at once. You could rig something up with the “Emit trigger signal” effect that puts out an array of units… but you’re right that there really should be a “Damage multiple units” helper since it’s a common thing to want to do. I’ll also make a note to add one.
Let me know if you run into anything else!