cookbook actionscript 3.0 pubblicato da o'reilly dice:

In Actionscript 1.0 and 2.0, you had many choices as to where toplace your code in the timeline, on buttons and movieclips, in external .as files referenced with #include, or in external class files.

Actionscript 3.0 is completely class based, so all code must be placed in methods of your project's classes.