Game concept
Tributary
The game also links it as Tributaries, Tributary's.
From the game's encyclopedia
A Tributary is a Ruler who has reached an agreement with another ruler, their Suzerain.
The Tributary provides a periodic tribute in the form of Herd, Taxes, Prestige or even Piety (depending on their type) and, in exchange, gains a Truce with the Suzerain.
Tributaries are a looser form of subject that cannot join Factions, but can break free if their requirements aren't met.
If the Suzerain is no longer Landed nor Migrating, the Tributaries will break free after a short amount of time.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Tributary is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Tributary is a Ruler who has reached an agreement with another ruler, their Suzerain." It is part of Subject. It covers Tribute Mission. The game also links it as Tributaries and Tributary's. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.