This is a feature in AS3 that I hadn’t come accross before. If you ever load swf files into one another you might not know it, but you’ll be dealing with application domains. I’ve loaded in SWF’s before, but never had to pay any specific attention to the applicationDomain.

Basically they allow you to specify where the class definitions for an SWF are sourced from. By default if two SWF files use a class with the same name, then only the definition found in the parent SWF will be used. If you want the child SWF to use it’s own version of that class, then you need to give it it’s own application domain.

From the Adobe docs:

The current project I’m working on loads in a variety of SWFs that extend a common base class. It ended up that I needed to make a change to the base class, so at first I thought I was going to have to go and recompile all of the external swf files in order to incorporate the changes I’d made. But actually, this wasn’t the case due to the default behaviour of the applicationDomain. It turned out that as long as that base class was used in the parent swf, then all the external swfs that were loaded in would use the defintion found in it and not their own. Way to go! It meant that I could tweak and make changes to those external swfs, without ever open or recompile them. It’s turned out to be really useful, and painless!