When I think of commentary, I think of an essay based on a key passage of the text. The essay mostly sticks to what's in that particular passage. A formal essay, though, pulls analysis from the whole text. It's the type of essay I wrote before IB essays. Just a normal essay that can be about anything. Not sure how clearly I described that, but the way I see it, if you want to write an essay on the author's use of X to do Y in a text, it's probably a formal essay. I never had much practice with commentaries (except for the IOC, I suppose) so I considered analyzing a poignant passage to do a key passage analysis, where the passage is connected to the rest of the text. I also considered doing a formal essay just because it was a normal essay. Does that answer your question?