The cardboard is provided to hide a disabled gang on the physical device.
Before attaching the faceplate, turn it, so you can insert the cardboard on the back. Choose which color you prefer to cover the gang and put it face down (we suggest to put the black side face down if you have a dark faceplate color, white side face down if you have a light faceplate color).
Insert the cardboard starting from one short side and put it parallel to the gangs in the space between the plastic and the glass as shown in the picture. Repeat with the other short side.
Move it on top of the gang you want to hide.
To see how to disable a gang click here.