"A vs B (and sometimes C) surfaces"

If you picture a chrome truck bumper,

the aesthetics of the nickel and chrome plated visible surface is important whereas the back side bolted against the truck is not visible, and looks don't matter.

Sticking with the truck bumper example, nickel-chrome plating is a very highly decorative finish, but it is extremely difficult to do a large surface like a bumper with zero or near zero "gas pits" ⇦ huh?

So picture having plated a bumper which came out perfectly on its top surface and front surface, but which has a few small gas pits on the bottom surface. Recognizing that nobody but a mechanic under the truck will ever see the bottom of the bumper, you might want to call the plating satisfactory.

That is the origin of the idea that some surfaces are more important than others. Many companies therefore assign "A" surface requirements to some areas, a laxer set of "B" surfaces requirements to other areas, and possibly even looser requirements to areas they consider "C" surfaces.