Objective-C中所谓的“类簇”到底是什么?

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?I was reading that NSArray is just such a thing