Flex 4 - What makes a GC root?

What are the rules for something to be a GC root in Flash?

I'm using the Flash Builder 4 profiler, and I'm finding odd things are labeled as GC roots in the object reference details (e.g., 开发者_运维问答an Image control that is no longer in the display list, nor static).

---

This discussion might be helpful.

http://tech.groups.yahoo.com/group/flexcoders/message/144288

Apparently there is some depth from the root at which the profiler just stops displaying the rest of the chain and labels the object as a GC root even though there is more chain above it.