I'm working on someone else's code. The application was architected as a large number of swfs that are loaded into a main swf as needed. The external swfs are all <mx:Application> rather than <mx:Module>. How is a swf <mx:Application> different than a swf <mx:Module>? I know you can't run a module independently. I'm more interested knowing if there say lots more code that goes into a compiled Application versus a Module. Does using modules positively impact performance? Reduce memory requirements?
Modules are much ligher than Applications and are meant to be separated out. Flash Builder by default puts all classes that are there in the application in the external library path of the module so that they do not get compiled into the Module as well.
See http://blogs.adobe.com/aharui/2007/03/modules.html for a good explanation.
P.S: You'll get better answers for this on the Flex General Discussions forum.