If you just click the GENERATE button, it generates parts for all the tracks. If you just want to generate a single part...

1) click on the track you want to generate. This puts focus on that track.

2) right click on the track, and when the floating menu appears, pick "SELECT AND GENERATE REAL TRACK"

3) if you only want to generate a portion of that track, then just hilight the part of the track where you want the real track generated THEN right click, and pick "SELECT AND GENERATE REAL TRACK"