The shotgun "zones," A, B and C, describing the shotgun's behavior at varying ranges, are not much emphasized in my practice sessions anymore, because it was always a clumsy teaching. It is easier to tell people that the farther away you are from your target, the more likely you are to pelt the downrange danger zone instead of putting pellets into your target. That is really all the zones have to teach us, and you can demonstrate the same lesson in a few minutes at the range. Here is how the matter was taught, and my critique.
Zone A: Very short range. The pattern has hardly spread. All your pellets will hit the combat silhouette target, for they are hitting en masse.
(Bad assumption. All your pellets can miss the target too--same reason. But, if you are reasonably proficient, it is quite likely that all of the shot charge hits--and the wad as well.)
Zone B The pattern has spread out, but not so widely that you can't still put all your pellets on the target.
(But imperfect aim will mean you hit with some and miss with some. A clearer way of saying it is that you cannot be sure they'll all hit but there's a pretty good chance.)
Zone C: Some pellets are certain to miss because the pattern is now larger than the target.
(You need to be very aware of the downrange danger zone. That is also a splendid idea when firing at the closer ranges. )
There should have been a Zone D: You are so far away that only by a sheer fluke will you hit your target.
(Buckshot will work for merely suppressive fire at 100 yards or more, but that is because people have a superstitious dread of "the one with your name on it." The odds of connecting are slight.)
Summation of critique: You should ALWAYS scan the downrange area. Be aware that the danger area is wider for a shotgun than for a rifle. But it is not much wider than for a pistol, because people shoot pistols so badly, especially when they are under stress. The shotgun zones were never more than a laborious elaboration of Rule Four.