Hmm. Looks like the game allows you to play basic Settlers and Seafarers, as well as C&K.
That's right. Campaign-wise, you start off with vanilla Settlers, play one round of the Harbormaster scenario, and then it's off to Seafarers-land for many, many scenarios. At the end, they bring it all together with some C&K.
As you advance in the campaign, individual scenarios are then unlocked. Some of them begin unlocked, too, just in case you have no interest in the campaign.
I got the C&K download and unlocked it...it's really damn good. I won the first scenario I played, but it was the "starter" thing. Great implementation, very polished. I bought it because I'm all the time sitting here saying that I want to play C&K more, now I can just do it whenever the hell I want.