The classpath

In order to use a class or interface that you've defined, Flash must be able to locate the external AS files that contain the class or interface definition. The list of directories in which Flash searches for class and interface definitions is called the classpath.

When you create an ActionScript class file, you need to save the file to one of the directories specified in the classpath, or a subdirectory therein. (You can modify the classpath to include the desired directory path; see Modifying the classpath.) Otherwise, Flash won't be able to resolve, or locate, the class or interface specified in the script. Subdirectories that you create within a classpath directory are called packages and let you organize your classes. (For more information, see Using packages.)