[AS3]Confused with as files

Firstly, let me say hi! I’m new to this forum although I’ve read many posts and found them to be extremely useful.

Now then, I’ve just started learning ActionScript 3 and I’m used to the old AS2 method of inserting actionscript into frames and movieclips etc.

I’m trying to learn how the *.as files work in AS3 but I’m confused as to how I can effectively use them when the fla file can only have one ‘Document class’.

I want to keep things organised and put some functions in one *.as file and others in a different one.