This question was posted in response to the following article: http://helpx.adobe.com/flash/using/draw-simple-lines-shapes.html
For anyone wanting to know a little more (or just see it in video), Brush tool modes are explored in this Flash Pro CS6 tutorial narrated by Geoff Blake of InfiniteSkills. He looks at Paint Fills, Paint Behind, and how to isolate painting to specific objects and shapes without affecting the areas around them:
Geoff also has a training video on using the PolyStar tool. We've been getting great feedback on this beginner series overall, so thanks for the support!