This button will ensure that the Thumbnail View, the Layout Popup, and the Mount View are displayed together in full screen on the monitor.
While in Full Screen, there's an option to switch between Layout and Image. There's also an ability to choose a capture device just like the Sensor Popup.
Display
It is possible to designate which display will show in Full Screen in Preferences > Digital Radiography > General and selecting either Primary Display or Secondary Display.