“K” at C180-K ??? What does that mean?

Hello am newcomer here and have no idea of Mercedes-Insider language. Now I am already in despair. I find nowhere what this K means at C180K or C200K. I only know that it is some extension. But what exactly, no plan. Please don’t be angry that I post such a banal post here just refers me briefly to an existing thread with detailed info. thx in advance!