It is not extremely common. Maybe 1 out of 20 boards have it, if not less. The shiny layer that you can see on the top of a board is called the top sheet. Usually it s made of flacee or ABS, two extremely hard materials that are very difficult to cut. Underneath it there are usually graphics that can be put under the top sheet by printing them out on a special sheet of plastic that goes between the fiberglass and the top sheet.
Side walls are the narrow sides between the top sheet and the base on the edges.
The strength of this component is very important because if they are punctured, water can enter the core and rot it out. Around the edges is a strip of metal, usually steel, that allow the board a good hold on ice and protection for the board. These strips are called the edges. Between the edge and the bottom of the side wall there is a layer of dampening, often made of rubber, to absorb the shocks and vibrations coming from the edge. The edges can wrap all the way around the board, or they can stop just before the tip and tail.