If nothing happend i noticed that deserve a commendation i usualy default to the "Pretty portrait" thing.
If a person was nice and added to the Content by just being social that can get a commendation.
If i noticed outstanding things in gameplay i also commendate that but that rarely falls onto DPS exception might be run saver like Red Mage.

If someone is rather unpleasant or even toxic in the social sense i wouldnt give commendation even if it other things apply. Its a KO Criterium.