Every symbol except the font symbol has a Timeline of its own that can hold text, video, graphics, and other symbols. How each Timeline works really depends on the type of symbol you're producing. Buttons, for example, can only display four frames per layer: Up, Over, Down, and Hit. Graphic symbols can have as many frames as you like, which means you can create animations in them, but a graphic symbol's Timeline is completely dependant upon its parent Timeline and therefore won't move unless its parent is also moving. Lastly, movie clip symbols have their own independent Timelines that can do anything that the main document Timeline can do. You can place movie clips inside of buttons, buttons inside of movie clips, or graphic symbols inside of movie clips and buttons, all to make more complex animations and interactions. You cannot, however, place a button symbol in a button Timeline or place a button instance inside of a graphic symbol because graphic symbols cannot be controlled with or contain ActionScript.
In Lesson 2, you learned about the edit bar. As you work more with symbols in this lesson, you'll find out how important it is to use the edit bar to navigate through documents that contain nested symbols because it tells you what Timeline you happen to be editing at any given moment.