领地

In Crusader Kings III, a realm is the land a ruler holds, either indirectly through vassals, or directly as personal holdings (their domain).

An independent realm is the highest political structure a character can be part of. It forms a hierarchy of landed rulers; a liege holds the highest title tier in the realm, and delegates land to vassals in exchange for taxes and levies. In realms large enough, these vassals may in turn have their own vassals, with this relationship repeating until one reaches barons - the lowest title tier.

Overextension

Dukes and Counts have a realm size limit of 30 counties. Each county over the limit gives an overextension penalty to Gold income by -10%.

Sub-realm

A sub-realm of a ruler is their domain as well as all of their vassal's lands.

Domain

The domain is the collection of all holdings a ruler owns personally. Each ruler has a domain limit; this determines how many holdings from which they can efficiently collect taxes and levies. Once the domain limit is reached, newly controlled holdings will not provide any taxes and levies and the ruler's vassals will receive an opinion penalty of -10 towards them for each controlled county that is over the limit. Domain limit is calculated by:

+1 per 5 points of effective stewardship skill (which includes any traits and spousal assistance)
+1 for each of the following cultural innovations: Ledger, Bailiffs, Guilds, and Court Officials (all in the Civic track)
+2 if Primary Title rank is Count or Duke
+3 if Primary Title rank is King
+4 if Primary Title rank is Emperor
+2 for "Divided Attention" in the Architect stewardship perk tree.
+1 for level 5 "Law" Dynasty Legacy