In campaign, once you hit silver+ in a commendation (not sure if this only applies to MAX RANK players or not), it stops giving you ANY cR for earning a new "point" in that commedation. Unless you get to the next level (say, onyx) in which case you earn the level-up cR kicker.

So, to give an example, say my Walking Tank is at the Gold level. I earn jack diddly shit for my double, triple, etc kills, unless the "point" I just earned made me level up to Onyx, then I'd get like 4K cR (give or take 1K) as a kicker for leveling up.

However, my Cannon Fodder may be Bronze, so I'll still earn cR for killing grunts (on top of leveling up). So if I kill 2 grunts and earn a double-kill, I'd only get 2+2 cR since, like I said, my Walking Tank is already at Gold and thus not ponying out any fucking cR.