If you are referring to the letters written on the cels
themselves, then I believe that refers to layers of cels
and not grades (quality). If you have a cel that uses more
than one letter then that is a multi-layered cel. I've seen
many cels which might have a A layer for the face and B layer for
the mouth for isntance.

The grading system of A, B, and C, however, are completely
different. You can search forum threads on this but in a
nutshell, these are ways collectors "grade" cels from high
quality to low quality.

A is the highest quality because it is a tight closeup of
a single character whose eyes are open and mouth is closed.

and so on... but these, of course, are subjective ^_^

Is this what you were asking?

Wendy