So... that's either UTF-16, an eight-bit encoding like ASCII or Latin-1, a variable-width encoding with an offset-translation table (as mentioned in the article

So... that's either UTF-16, an eight-bit encoding like ASCII or Latin-1, a variable-width encoding with an offset-translation table (as mentioned in the article), or a string with a different encoding but a constant upper bound on its size. (Or a few other things that seem less practical.) The first two are used by CFStringRef (bridged to NSString), the third by Swift strings that have been hit with UTF-16 offset lookups, and the fourth by Objective C tagged pointer strings (on 64-bit devices). Yeah, I guess that NSString has mostly exhausted the possibilities for ways to fulfill their API obligations.