I was reading that NSArray is just such a thing. Sounds heavy. I have 7 really fat books here on my desk about Objective-C, Cocoa and C. None of them mention Class Cluster at all, at least I can't find it in the Index at the back of the books. So what's that?
From Apple's docs.... In short it's a design pattern used in the Foundation framework, which is probably why it's not mentioned in ObjC books.
A class cluster is an architecture that groups a number of private, concrete subclasses under a public, abstract superclass. The grouping of classes in this way provides a simplified interface to the user, who sees only the publicly visible architecture.