From an ActionScript perspective, each Movie Clip symbol instance in a .fla file is represented at runtime by an instance of the Sprite class or one of its subclasses. The class used to represent instances of a specific Movie Clip symbol is known as that symbol's linked class. A symbol's linked class can be specified manually or generated automatically.
To set the linked class for a Movie Clip symbol, we use the Linkage Properties dialog. Note that if any of the following are true, the specified linked class must inherit from flash.display.MovieClip:
The symbol's timeline contains any frame scripts.
The linked class wishes to control instances of the symbol programmatically, using MovieClip methods.
The symbol's Stage contains any components with customized parameters and either of the following is true:
The customized parameters are not identical on all frames of the timeline. For example, a Button's label is "OK" on Frame 1 and "Submit" on Frame 2.
The component does not appear on all frames of the timeline. For example, a List with a custom data provider appears on Frame 1 but not on Frame 2.
The symbol's Stage contains any components with customized accessibility properties or Strings Panel content.
Otherwise, the specified linked class need only inherit from flash.display.Sprite.
Here are the steps for specifying the linked class for a for a Movie Clip symbol:
Select the symbol in the .fla file's Library.
Select the pop-up Options menu in the top-right ...