The "control area" is used to access the system and switch programs.  After pressing here, you will see the standard PocketPC control bar at the top of the screen.  To switch back to full screen mode, press on any other portion of the screen (perhaps avoiding the buttons).

The "info area" brings up a dialog box showing a photo of the back side of the original HP-35 calculator as shown below.  Press the "ok" button (located in the top-right corner of the screen to return to the calculator.

The "on-off area" is used to terminate the program.  Tap here and a dialog box will confirm your selection.