IntrinsicClass : Object
It's not possible to determine if an object is an IntrinsicClass object using x.ofKind(IntrinsicClass) or via x.getSuperclassList(). This is because those methods traverse the nominal class tree: [1,2,3] is a List, and List is an Object. However, List and Object themselves are represented by IntrinsicClass instances, and it's occasionally useful to know if you're dealing with such an object. That's where this method comes in.
This method returns nil for instances of an intrinsic class. For example, isIntrinsicClass([1,2,3]) returns nil, because [1,2,3] is a List instance. If you get the superclass list for [1,2,3], though, that will be [List], and isIntrinsicClass(List) returns true.