The review question at the end of Hooktheory I 4.1 uses the term "harmonically compatible". Where was this concept introduced or explained?
I initially thought it was the same as stable notes but it doesn't seem to work the same way.
Sorry for the delay in getting back to you.
Harmonically compatible means that the chord would sound good there; it has to make sense with both the chord progression it is a part of and the melody above it. This chapter talked about stable and unstable notes in melodies and how they resolve so that is one thing it should follow. In that particular example, the iii chord worked with the melody but wasn't as compatible with its neighboring chords in position (C) ((C) was a chord that needed to resolve to I at the end of a progression and a iii chord doesn't work in that context)