fred can take his lands castles whatever to another kingdom if he gets tired of being part of fredland. or joe can tell fred to bugger off and fred will be on his own with his lands or castles or whatever.
but you can also grant manager-ships to parts of an ek with other vassals so maybe you only have a manor house, but you are the barron of a bunch of other lower vassals in the EK you can be fired from the ek, you take your one plot with you but the rest of everyone stays.
From the FAQ
Parcel ownership, however, is not subject to the fealty chain. That means if you own a parcel, you can be evicted, but your parcel can never be taken from you. If you have a disagreement with a monarch, you also have the option to pick up your parcel and move it even to another kingdom.