I need a little help understanding what a particular exercise is trying to teach me. For reference, this is in the Phil DeGreg Jazz Keyboard Harmony book, exercise 2-4A. It's labeled 'Diminished / Tri-tone Sub'.
The first line has the guide tone chords listed for CM7, C#dim7, Dm7, D#dim7, Em7, Eb7, Dm7, Db7, CM7.
While I can see that some of these chords are intended to be diminished substitutions, what are they substituting for?