Does no one read the actual definitions anymore? It's on the website, ya know.
C means that he has potential to hit his number, but depending on how some qualities turn out he could drop 2 points in his potential rating (ex// an 8.0C means that at best they think he'll be a 1st liner, but he has a chance of turning into just a 3rd line forward 6.0 or anywhere in between).
B means that he's quite likely to hit his potential, but at worst could drop a single point from his rating. For example an 8.0B means he's got potential to be a first liner, but if that doesn't pan out fully he still could be a 7.0 player, or a second liner.
Connor McDavid potentially being a 10.0C means that he could be a generational player, but at worst he's a 1st line center 8.0, which is pretty solid and fair IMO. If he never becomes a Sidney Crosby, he's still good enough that he should be able to play a role on the top line of most NHL teams and score 65-70 points at his peak, the definition of an 8.0 forward.
IMO he should be a 9.5C at the least.