MovieClip inside another MovieClip for a drawing applet. Why and how?

Question

I'm trying to figure out a simple drawing program in Flash and it's giving me trouble. The main drawing area is a MovieClip named drawZone. However, when displaying something, the author uses another MovieClip embedded in the drawZone Symbol named zone. The drawZone Symbol has 2 layers, a mask layer and a masked sublayer that contains the zone Symbol.

1. Why does the author do this instead of simply drawing everything in the drawZone MovieClip?
2. The only place the zone Symbol shows up is in the Movie Explorer. Where do I access it, and how would I add such an embedded Symbol to my own code?

Here's a screenshot.

Answer 1

My Flash skills are a little rusty, but I think it's because you can't apply a mask to the root movie.