Fief

A fief is property that is granted to a noble in a special legal document called a letter of enfoeffment. Typically, when a world is admitted to the Imperium, a soverignty certain portion of it is transferred to the Imperium. The Imperium then grants portions of this land to become fiefs of new nobles associated with this world.
A fief is not just a place to build a mansion, it ties a noble to the world that falls under their title. Fiefs are not required to be land. They can also be wilderness areas, industrial facilities, blocks of property in a startown, or any combination of these. More complicated fiefs can include stocks in a business, the right to collect taxes on certain industries, and so on.
Even when a fief consists only of land, the fief itself may be divided across different locations on a world, but are virtually never across different star systems. In systems that do not have habitable worlds, a fief may include part or whole ownership of a space station, asteroids or planetoids. The Imperial noble system does not include subinfeudation, so they do not give portions of their fief to a vassal.
Nobles are expected to take care of their fiefs in order to ensure that the noble can maintain their station in the Imperium and to keep them in good condition for the next holder of the title. Nobles who do not keep their fief in good condition can have the fief stripped from them, and in more servere cases they may also lose their noble title. A fief is attached to the title and cannot be sold, transferred or inherited by offspring, on whole or in part. Any hereditary title that is inherited by a noble's heir also comes with the fief that is attached to the title.
Nobles can also own property and wealth like any other citizen, even if income from a fief is used to purchase it. This personal property is like any other personal or family property and may be sold, have a lien placed on it, be divided up in an inheritance, and so o0n. Also, this personal or family property cannot be taken if a fief or a title is stripped from a noble.