The Estates of the High King are those used by the family of or office holder of the title of the High King of the Steel Realms. They are used by the family as a vacation/travel or safety destination and there are no lands, gratuities, or services associated with it other than the estate itself.
There are many Estates of the High King. Each duchy maintains one. Each Grand Barony and Grand Duchy assists with the costs to maintain them when empty. The high King’s offspring, siblings and his nieces and nephews serve on these estates with the local soldiers gaining experience as squires and pages and even when they become Knights sometimes running these small estates and households. But never for a period longer than 2 years which the charter then requires them to relinquish and return to the high Kings principality of Oerdney for a new mission (often all are just rotated through the estates).
All the estates, no matter where they are, are considered Dominion of the High King Lands.