Ring Chain
What is the difference of a Dura Ace 53T chain ring A Type or B type?
Dura Ace Chain ring difference from A type or B type
It is more to do with compatibility - an 'A' big chainring should ideally be matched with an 'A' smaller chainring and likewise with 'B' sets.
This is all to do with the special 'Superglide' system designed to make front shifting easier by means of special shaped teeth on the chain rings that allow for smoother shifting.