Thanks Matt,
Your explanation clarifies my understanding of the different tracks.
I tested it myself.
Some Melodist and Soloist tracks can also be RealTracks.
When I add a RealTrack Melody (a fiddle eg) to a song, the term "Melody" is replaced by "Fiddle" in the Mixer and at the top of the screen.
When adding a Midi instrument, the term "Melody" remains unchanged.