The two edges- the base edge and the side edge- are both the same piece of metal. The base edge is on bottom of the ski, the side edge is on the side. What cap construction refers to is the way all of the pieces of the ski are bound together- a "cap" topsheet (the part with the graphics on it) is actually placed over the top of the entire ski and it extend down the sides of the ski to the edges. On a "sandwich" construction ski, the topsheet is exactly that- the very top sheet of the ski, the part with the graphics on it. Instead of the topsheet extending down to the edges of the ski, there is a special sidewall material bewteen the edge and the topheet.
Here's the best picture I could find:http://www.biglines.com/photos/blpic17252.jpg
The fingernail scraping are sitting on the base edge, the side of the same piece of metal which is exposed on the side of teh ski is the side edge. This particular ski is of sandwich construction; the blue side part is the sidewall.