I heard one coach describe sectioning the strike zone into quarters - and then trying to section each quarter into additional quarters - such that you end up with 16 "spots". I suppose every pitch is not meant to be a strike so I assume there are targets outside of the strike zone, but not sure how those were incorporated into the 16 sections.
Assume width is 18 inches (technically strike zone is 17 inches) and height is 36 inches (pretty tall), that gives you 648 square inches. 16 sections each comes out to be a box approximating (big time rounding here) 4 inches wide and 8 inches tall. Most, if not all, similar pitches (i.e. 4 seam FB) have identical mechanics with the difference being largely, if not exclusively, the release point. And the difference in release point between two adjacent boxes would seem simply too small to appreciate and replicate. That said, there are folks out that that can, and do, hit individual boxes at will.
My guess is that is has something to do with "touch" and a pitcher's ability to fully appreciate his physical presence and know where he is in space - to a very high degree. Sounds kind of mystical, but assume it can be taught.