Game concept
Holy Order Patron
The game also links it as Patronize, Holy Order Patronage.
From the game's encyclopedia
A Holy Order's Holy Order Patron is a Ruler of Duke-Tier or higher who shares their Faith and whose Realm contains their Capital. A Patron must take care not to lose the County that contain this Capital, as patronage can be lost to Vassals of Duke-Tier or higher.
Patrons may either be a Monastic Order Patron (of a Monastic Order) or a Military Order Patron (of a Military Order).
Not all Holy Orders have a Holy Order Patron, and they are not strictly required to.
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.
Holy Order Patron is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Holy Order's Holy Order Patron is a Ruler of Duke-Tier or higher who shares their Faith and whose Realm contains their Capital." The game also links it as Patronize and Holy Order Patronage. 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.