Flash has more than one timeline

The Main Timeline: The most obvious, this is what you see when you open Flash.

Scenes: which are really just a continuation of the Main Timeline.

Symbols: movie clips and graphics have their own timelines

The reason for all these timelines is that they organization your project and allow for different animations or programming to exist on their own particular cycles. This also allows for nesting (putting one symbol within another) to make controlling and animation happen to more than one part of your movie at the same time.

Movie Clip Timelines

Movie Clips are particularly unique in that you can put sound and programming inside or attached to them. You cannot do this with graphic symbols, they cannot use programming. You can also put programming and sound on the Main Timeline and in Scenes.

You can also put sound in Buttons, and you can put Movie Clips in Buttons, and you can put actions on buttons (such as On Press go to Frame 3), but you cannot put script on a frame inside a button's timeline.

Movie Clips can be targeted with programming,meaning that you can name a Movie Clip and then tell it to do something calling it by name. This is also called addressing: the act of targeting specific
movie clips throughout your movie.

Movie Clips must be named to target them. You can have many instances of a Movie Clip on the stage at the same time, you give them unique names to tell them apart. It gets more complicated when one movie clip is inside of another movie clip on the stage. You have to know where you are when you are asking the movie clip to do something and then figure out what the names are for the nesting path.

Addressing and Targeting

note: All variables that you set in one movie clip or timeline, live only in that timeline, unless you specifically send the values to another timeline.

Movie Clips have a few unique properties:
1. It will begin to play as soon is the frame that it lives on is reached, even if the main movie (timeline) is not playing.

2. A movie clip plays autonomously, meaning that as long as it is present on stage at the point that the playhead is on it will play, it is not governed by the playing or stopping of the main timeline.

3. All timelines are subject to the frame rate chosen in the Modify/Movie dialog box. Although it is possible to control a timeline's frame rate with actionscripting.

example. If the main timeline has a stop action on the first frame, then all three movie clips will continue to play unless the have stop actions on their first frames, or they are told to stop by targeting them.

Don't confuse symbol names in the library with instance names on the stage. An instance is one occurrence of a symbol on a stage. You can have as many instances of a symbol on the stage as you want.

There are two types of ways to address/target a movie clip.

Absolute Addressing: which starts all targeting paths from the main timeline and proceeds from the location of the main timeline.

Relative Addressing: which starts the targeting path from the location that the action is being called from and not the main timeline.

Relative addressing works well with duplicating movie clips or movie clips that you want to work anywhere you choose to place them, even in other Flash files. In general if you move things around this is best for you. If you like to reuse your code again it's indispensable.

You can use the Target Editor in the Actions panel or through the Behaviors Panel to choose your target if it is available at that point on the timeline.