Game concept
Order Vicinity
From the game's encyclopedia
The minimum range at which a Monastic Order or Military Order is accessible to a Ruler.
To satisfy Holy Order Vicinity, the Order must have a Leased Holding that is within your sub-Realm or a neighboring County.
Military Orders must be within Holy Order Vicinity to be hired for Wars. Similarly, most Monastic Order Interactions are not available while the Monastic Order is too far distant.
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.
Order Vicinity is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The minimum range at which a Monastic Order or Military Order is accessible to a Ruler." It is part of Monastic Order. 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.