What you're training
A chord and a melody line don't have to agree note-for-note — a lot of what makes a melody sing is exactly the notes that aren't in the chord underneath it. This drill holds a plain C major chord and plays a short melody on top, and your job is to hear whether that melody stayed inside the chord or slipped in a non-chord tone — a note that belongs to the line, not the harmony.
Two levels, one skill
Level 1 asks the basic question: clean, or decorated? Level 2 assumes you can already hear the decoration and asks which kind it was — a passing tone (stepping through from one chord tone to another, E-F-G) or a neighbor tone (stepping away from a chord tone and right back, E-F-E). Same three notes, two very different jobs.
Go deeper
The notes the reduction threw away — 660 recorded drops → shows just how often real melodies lean on notes outside the underlying harmony. The theory: embellishing tones → covers the full family of non-chord tones and how each one behaves. A different kind of melodic detail → trains the same close listening to a melody line, just for its scale color instead of its relationship to the chord underneath.
Where it fits
Comfortable with chord quality already? Non-chord tones is the melodic side of the same coin — the chord stays put, and the skill is hearing exactly when the melody steps outside it. The full drill ladder has everything else.