Version 3.5.8
In the very middle of the edit window, you'll see a small "LED" on the far right (it is red in the example above).
The intent of this small light is to tell you whether you are editing a shot which is LIVE or not.
The color can be one of:
Red -- the shot you are editing is currently Live in the broadcast
Yellow -- the shot you are editing is currently in Preview (if you press "Go" it will be Live)
Clear -- the shot you are editing is neither in Live nor in Preview