The more useful answer is:

a) it needs namespaces

b) but giving people namespaces is unironically bad because it's what lead to "enterprise development" style APIs like C# where everything is named System.DataStructures.Collections.Arrays.Lists.ArrayList, as if giving something a longer name makes it more professional.

c) so instead two letters means a system framework and three letters means a user framework

I quite like a terse but consistent conventions myself. I remember finally being able to quiet the tedious part of my brain that couldn't get past the NS conundrum when I finally came up with the NextStep thing as a reasonable theory.

In other words, my only complaint is that this Apple convention is not more easily discoverable. Or perhaps that the expert author of the book I was reading (this was back in the day) didn't feel the need to share it with his readers.