In the last part of Chapter 3, we learn about musical addition, subtraction, context, and subfigurations!
Addition & Subtraction
When you add tones (e.g. passing, neighboring, anticipation, ornamentation, and/or auxiliary tones) to an original theme, this is called Addition. When you delete tones from your theme (and replace with rests and/or lengthen previous tones), this is called Subtraction.
Changing context in the musical sense is not so different from changing context in other realms! You can change the "context" of a melody by reharmonizing it (changing the chords), using different orchestration, tempos, dynamics, articulation, etc.
After a little poking around on the interwebs, it appears that the term subfiguration isn't widely used in the lexicon of composerly terms, but is essentially breaking a theme into separate identifiable figures that you can then transpose, reiterate (etc.) to create new material.
I had an inordinate amount of fun exploring these transformation tools by creating a theme, chopping it up into smaller figures, improvising at the piano, etc. Definitely a song-seed for my next album. Happy listening!