Property cases (PropertyCase or PropCase) are basically variables in the camera's RAM that we can read and write from CHDK by using uBasic commands get_prop and set_prop. "Under the hood" some other uBasic commands work by changing propcases, making it possible to use them in a portable way. Some propcases are read-only. Often writing to a propcase will have a different effect than pressing the corresponding buttons on the camera's user interface. Typically this means that while you may change something like the shutter speed and your change will be effective in your photo, it may not be updated to the camera's on-screen display.