There are three pieces, they are not attached but are captured by the wheel wells and tailgate so they cannot move. 3 pieces because you cannot make it long enough and wide enough in back out of one sheet, and it is much easier to remove and install and store in 3 pieces.
The rails on the sides are wood blocking with polyethylene rub rails. They are to keep the camper centered, and are approximately 1/2" wider than the camper. I had trouble with movemet on the new camper. I had been told the bed liner was a bad idea, allows the camper to rock and move. On my old truck I removed the bed liner and had Line-X installed. No difference at all, other than my wallet was $500 lighter. On the new truck I went back to the bed liner. The plywood platform keeps the camper from moving, and also provided plenty of bearing on the bedliner so there is no rocking. I've never tried the camper right on top of the bedliner, always had plywood in there.
From experience, the poly bedliner protects the bed better than the Line-X, which is why I went back to it.