This menu appears when a scene is opened. It displays all of the cast members for the selected scene. It lets you make a cast member the current cast member by selecting its picture from the menu. After tearing off the menu, you can double click a cast member to select its window (and run if it is a moving cast member).
If the picture is a moving cast member in the lower left coner of its box you will see one or two letters. The first letter tells you what type of run it is (ie. n for a normal type run). The second letter is 'r' for reverse if that run is a reverse run. See topic Animation Control Panel for more info on runs.