I assume the difference between a rank 6 farmer and a rank 7 farmer is that a rank 6 farmer pays rent or corvee labor (or is squatting), but is free to leave, while a rank 7 farmer is bound to the land, and gets only what the landlord gives him. I've seen the word peasant used for both of those statuses, though, and I probably would leave it out of the description of rank 7. I think serf gets the idea across better.