Game concept
Tributary Contract
The game also links it as Tributary Contracts.
From the game's encyclopedia
Tributary Contract dictate the Obligations between a Tributary and their Suzerain.
All Tributaries have access to different Obligations, negotiated individually for each Tributary.
Tributaries pay a percentage of their total Taxes, Levies, Herd, Prestige or even Piety to their Suzerain, based on their Obligations.
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 Contract is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Tributary Contract dictate the Obligations between a Tributary and their Suzerain." The game also links it as Tributary Contracts. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.