If you place it in a corner, you may have glass on two sides. Place it in a wall, and you'll only have one side open. I think they are referring to which sides have a curb...the remaining sides are designed to be either tiled or to have their panels installed.
Some people love receptors, but a well done tiled shower doesn't have to be a maintenance nightmare. Check out www.johnbridge.com and www.schluter.com for some ideas.