Asset and project config preloading

I’m looking to implement a configuration and asset loader and have come upon the AppCoreLib. 

On it’s face this it like a useful contribution but I’m unclear as to usage details. For example, in the sample source we see tags for both <swfs> and <urls> but nothing for image formats (gif, jpg, png).

It’d be very useful to understand the intent of ‘<urls>’ in both content.xml and config.xml.