Game concept
Military Order Patron
The game also links it as Patronize.
From the game's encyclopedia
A Military Order's Holy Order Patron is a Ruler of Duke-Tier or higher who shares their Faith and whose Realm contains their Capital.
The Holy Order Patron may call upon the support of the Holy Order against religious enemies for free, instead of paying Piety.
Patronage can be lost to Vassals of Duke-Tier or higher, if control of the Order's Capital is ceded.
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.
Military Order Patron is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Military 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. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.