If two tracks are grouped for Mix, then changing the volume in one makes the same change in the other. The Fades dialog box will appear. You can view the fade with or without the sound waveform. You can shade the fade by click-dragging with the mouse. Once you have what you want, click Ok and the fade will appear on the region.
Volume is automation and can move independently of the region.
It can be slipped foward or backward on a track. It is infinitely flexible, and therefore riskier, especially as you build up tracks and relationships between tracks. Experiment and learn. The other nice way to handle volume is in real time by clicking your mouse on the volume button, and dragging it up or down as the piece plays. It's a good way to rough out a track and then go back in with the little finger icon and get anal, as it were.