From the game's encyclopedia
A Rite is a branch of a Faith: a local variation of its religious practices, led by a Head of Rite and followed by its own Counties and Characters. Every faith has a Mainline Rite, and may grow several more.
A Rite defines itself by the Tenets and Doctrines it champions. Whether a Tenet is Permitted or Prohibited is judged by the Faith through its Mainline Rite, and the more a Rite's choices defy those judgements, the higher its Divergence: divergent Rites sap the Faith's Fervor, come to be seen as Heresy, and at 100 Divergence break away as a Faith of their own.
New Rites may be founded by the pious, Rulers can be swayed from one Rite to another, only the Head of Rite may edit one.
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.
Rite is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Rite is a branch of a Faith: a local variation of its religious practices, led by a Head of Rite and followed by its own Counties and Characters." It is part of Faith. It covers State Rite, Mainline Rite, Head of Rite, Divergence, Tenet Status and 1 more. The game also links it as Rites, Rite's and Rites'. 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.