The document class is like any other class except that instead of being for an object inside a movie it is the code for the movie itself.
Experiment: Download the movie
INDEX, Introduction to Using Classes, Preferences: Finding the Class Files, Spinner Class, Using the Spinner Class: Demo, Gravity Bounce, Water Class, Clicker Clown, The Document Class, Create New Movie with the Navigation Class, A Puzzle Document Class, Puzzle Demo, The Bear Puzzle