"About targeting older versions of Flash Player

Deciding which version of ActionScript to use

If you are creating content for Flash Player 6 or Flash Player 7 and are implementing custom classes and objects, you should use ActionScript 2, which supports object-oriented programming techniques in a more standard way than was done in earlier versions of Flash. If you are creating content for Flash Player 5 or earlier, you must use ActionScript 1. To specify the player and ActionScript version you want to use when publishing a document, select File > Publish Settings and then make your selections in the Flash tab.

If you have never programmed in Flash before and are targeting Flash Player 5 or earlier, see Object-Oriented Programming for Flash Player 5.