ActionScript is internal to Flash, the end users won't know the difference. When you set your publish settings to actionScript 2 or 1, and you attempt to publish the SWF, Flash will warn you if there are actionscript 3 functions that are not supported by actionscript 2.0. Otherwise, it will convert or use the code to compile the SWF.
It's a little different for the Flash player version in publish settings. If you set it to some old version compatibility you may get a similar warning and will have to adjust the publish settings to a higher version. But if, say, your SWF requires the Flash 8 player and a visitor has Flash 7 or older, they will get a prompt to install the latest version to see this Flash.