ObjC category naming convention

learning some sample code from the iOS programming course (cs193p fall2010) that I came across the sbjson structure that extends NSObject, etc. by category SBJSON. but in the header + "NSObject + SBJSON.h" it reads like:

@interface NSObject (NSObject_SBJSON)

where does the magic mapping of NSObject_SBJSON to SBJSON come from? I noticed that xcode accepts any line before underscoring !?

but I haven’t found a hint anywhere.

Thanx klaus