The "UP" label is so the image is right side up at the time of installation. It can be flipped electronically in the GUI of the camera. For example let's say you have an area that is relatively long but narrow. The "UP" indicator would let you set the ball at 90 degrees from normal fairly accurately, then the image can be electronically rotated to achieve "corridor" or portrait mode. If the view is a normal FV, wider, the up lets you get the image pretty much aligned at the time of installation.
The rectangular opening, with two Philips screws, is an access plate for the SD card slot and the reset button.