Chris, the Settings window does not give more detail on the harmony notes. It allows you to set things like harmony volume, voice harmonized, etc. It's the same for all harmony types.
Luluc, I did what Derek suggested, and recorded a midi of 8 notes of each harmony type with a chord of C. Here are the results:
C D E F G A B C (melody)
E G G C C C E E (Duet1)
E G G A C C D E (Duet2)
As you can see, Duet1 will always harmonize with a note of the chord, Duet2 will sometimes use other notes, especially if it can do a 3rd.
Cheers,
Fred