I've been nerding out on the Hal Galper mater class you tube vids. He mentions "superimposed triads" a few times and i thought I'd look into them. It appears to be a common thing in jazz piano. what i have gathered is this: these are triads based on the scale of the chord that are made of upper partials. so a Cmaj7 chord (C E G B) might have D maj triad (D F# A) superimposed on it and get a Cmaj7#11 chord...I guess.... If that's pretty much it, then where do these superimposed triads come from? That is, which of the various possible triads does one choose and why? are they just "as charted" or is the piano player pulling these out on the fly? I guess they just seem rather arbitrary to me. any rules of thumb for their use, similar to when you'd use a tritone sub?