This is the Overflow button. It is only available on the touchpanels in Rooms W210, W220, and W230. With it, you can display video from the Auditorium on the screen in the room. Press the button and the lights will dim and the screen will lower.
The left side of the touchpanel contains the Send to Projector button and display screen. When the Overflow Button is pressed, the display screen will show what is currently displayed from the Auditorium. However the projector will not display the video until the Send to Projector button is pressed. When this button is pressed, the screen will descend and the projector will activate. Until then, the video from the Auditorium will only display on the display screen of the panel.