When you create a movie clip instance in a Animate document, the movie clip has its own timeline. Every movie clip symbol has its own timeline. The movie clip’s timeline is nested inside the main timeline of the document. You can also nest a movie clip instance inside another movie clip symbol.
When a movie clip is created inside a Animate document, or nested inside another movie clip, it becomes a child of that document or movie clip, which becomes the parent. Relationships between nested movie clips are hierarchical: modifications made to the parent affect the child. The root Timeline for each level is the parent of all the movie clips on its level, and because it is the top-most Timeline, it has no parent. In the Movie Explorer panel, you can view the hierarchy of nested movie clips in a document by choosing Show Symbol Definitions from the panel menu.
To understand movie clip hierarchy, consider the hierarchy on a computer: the hard disk has a root directory (or folder) and subdirectories. The root directory is analogous to the main (or root) Timeline of a Animate document: it is the parent of everything else. The subdirectories are analogous to movie clips.
You can use the movie clip hierarchy in Animate to organize related objects. For example, you could create a Animate document containing a car that moves across the Stage. You can use a movie clip symbol to represent the car and set up a motion tween to move it across the Stage.
To add wheels that rotate, you can create a movie clip for a car wheel, and create two instances of this movie clip, named frontWheel and backWheel. Then you can place the wheels on the car movie clip’s Timeline—not on the main Timeline. As children of car, frontWheel and backWheel are affected by any changes made to car; they move with the car as it tweens across the Stage.
To make both wheel instances spin, you can set up a motion tween that rotates the wheel symbol. Even after you change frontWheel and backWheel, they continue to be affected by the tween on their parent movie clip, car; the wheels spin, but they also move with the parent movie clip car across the Stage.