用于绘图小程序的另一个MovieClip内的MovieClip。为什么以及如何？

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.I'm trying to figure out a simple drawing progr