I think you're confusing the common "A-B-C" 
terminology in actually 'grading' cel 'quality' 
versus the A-B-C-etc. in cel layers.. The 
lettering in cel layers has nothing to do with 
quality, but are related to how a sequence is 
animated. The actual 'grading levels' have been 
discussed here before, and can be found in the 
archives... In terms of value, the ABC grading 
scheme is a bit controversial but considered 
standard in Japan, I think.