When you click on the ‘R’ some fairly unpredictable things can happen.
For example, you may have a track where you -had- Volume automation, but turned ‘R’ off, left it off for a few days, forgot about yer old automation and changed the fader level. You hit ‘R’… now your old automation is back, but the fader level is lost.
Oops. That’s bad.
You should be able to hit Undo and restore the fader level you just lost as well as deactivate the ‘R’.