From the game's encyclopedia
An Interaction that allows a character to buy Unpressed Claims in exchange for Piety.
• A Vassal may not buy a claim on any Liege's Title, and a Liege may not buy a claim on any Vassal's Title
• A King or Emperor may not buy claims on Kingdom Titles, and Emperors may not buy claims on other Empires
• No one may buy a claim on their Head of Faith
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.
Buy Claim is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Interaction that allows a character to buy Unpressed Claims in exchange for Piety." It is part of Interaction. The game also links it as Buy Claim Interaction. 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.