Game concept
Mainline Rite
The game also links it as Mainline Rites, Mainline.
From the game's encyclopedia
The Mainline Rite represents the most prominent set of beliefs of a Faith. A non-Mainline Rite is considered to be Divergent from this Rite.
Which Rite is Mainline depends on whether the Faith has a Head of Faith.
If the Faith has a Head of Faith, the Mainline Rite is whichever one they follow. This means it can change when a successor of a different Rite takes their place, including a potential Challenger Head of Faith.
Without a Head of Faith, any Rite followed by more Counties than the Mainline supplants it, provided it holds at least as many Holy Sites as the Rite it seeks to replace.
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.
Mainline Rite is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Mainline Rite represents the most prominent set of beliefs of a Faith." It is part of Rite. The game also links it as Mainline Rites and Mainline. 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.