That's not what I meant. I was using the duke as the highest example of someone under subjection.
The king, obviously, owns all the land. Let's say he rents it out to 3 dukes. Those dukes must each pay a certain amount to King Squideey. Now, one of these dukes rents out some of his land to 2 counts. Now they must pay the duke a certain amount to help the duke pay the king. Anyone can have any amount of vassals under him, helping him pay HIS share.
So we start with a king, he has dukes, his dukes have counts and they have barons and the barons have knights, and the knights have serfs. The serfs can also have their own serfs under them. Serf is simply referring to the bottom of the food chain.