Nope. These answers are wrong. Neither of these action have an effect. Is this the second major S1 bug I've discovered? All of a sudden, the piano roll shows a jillion notes from a jillion tracks. I never did anything to make it do that. It's happened several times.
Selecting a track on the left hand list SEEMINGLY makes it active, but all the other freaking notes are still there. This is no way to work.
Double clicking on the track just makes the roll appear with a whole crapload of tracks selected.
And when I email support with bugs and problems, their response: "We are not here to teach you this program." Holy Hate-Your-Customers, Batman!
BTW, restarting S1 doesn't fix it either.... it's random... like other random bugs I've encountered.