Before Plone 3 these were called Smart Folders. Now we call them Collections. Since this project is Plone 2.5.4, I guess these are called Smart Folders.
So what I'm doing is extending it to include a few extra fields. Its for a last gasp requirement change. Making the content module was not much work. Making easily removable view changes across 10 odd views has been a bit more work. But I think I'm done.
Anyway, it was easy to do.