I don’t feel qualified to explain voice allocation–I understand it in my head, but doubt I’d explain it well.
Unison is well…unison : ) It uses all the voices to play the same note. You can use the Unison Detune to make a thicker sound. This detunes all the voices slightly so you get more harmonics (?). If you look up a basic definition of Unison it may help haha. In this mode you play only one note at a time on any of the tracks (it essentially turns the A4 into a monosynth). A lot of dance music uses unison voicing modes for big lead sounds as they tend to be “thick” and stand out in a mix.