The common.swc is a merged swc of all dependencies of the application. The swc file format is just a zip file with a manifest of its contents. Fabrication based applications have 2 dependencies, PureMVC multicore and PureMVC pipes. I have merged these together into one swc for easier development. If you are interested in how to do this, check out the fabrication build system. To do a merge of multiple swcs into one use,
<include name="name.of.swc" />
The implementation only works with simple swcs. Swcs that have additional assets and graphics don't work well with this macro. Also the merge is not at the swf level. The merged swcs contains multiple swfs, so its not ideal if you use runtime shared libraries and need a common swf.