Game concept
Subject
The game also links it as Subjects.
From the game's encyclopedia
A Subject is any Ruler that has sworn loyalty to another ruler. They can be of various types, like Vassals or Tributaries, each with their own specifications.
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.
Subject is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Subject is any Ruler that has sworn loyalty to another ruler." It is part of Ruler. It covers Tributary. The game also links it as Subjects. 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.