The C number seems to be taken directly from the results they get, like:
CMD16GX3M4A2800C11 = Tested Latency 11-14-14-35 ($839.99)
CMD16GX3M4A2800C12 = Tested Latency 12-14-14-36 ($729.99)
I guess we know which one Corsair thinks is better. However, sort their store by speed and it seems that overall, less latency = lower MHz, and lower price. So between otherwise identical things, a lower Tested Latency is better (they all seem to have the same SPD Latency in the Dominator Platinum section), but at the same time they seem to be suggesting we look at MHz and GB first, by rather a lot.
Not that I know anything about latency in RAM, this is just what I gather from Corsair's pricing strategy. ^_^
Naturally, I'd not suggest you actually pay anything like $110 for the odd spare nanosecond unless you bathe in money, and I'm unsure you'd ever perceive the difference between something like these examples...