Optional pitching rules mean that, like in the basic game, pitcher’s grades can be improved or reduced based on good or bad performance.
For example, if a C pitcher allows no earned runs in any six consecutive innings, his grade is advanced to B.
Or if any pitcher allows five earned runs in any three consecutive innings, his grade is reduced by one after the fifth earned run is scored.
Edited to add: What HOFfridge just said, a few minutes before I did. :D