From the game's encyclopedia
An Interaction which facilitates the peaceful transfer of a Title or Vassal between two Rulers, relying on the acceptance of the target Ruler and a secondary intercessor.
Refusal will usually cost the recipient Piety and Legitimacy, as well as an additional cost selected by the actor as a method of compelling acceptance.
The intercessor, a relevant Ecclesiastical or Theocratic ruler, may both forbid this interaction from even taking place and, with the Absolute Church Authority Law, may force through its acceptance.
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.
Negotiate For Land is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Interaction which facilitates the peaceful transfer of a Title or Vassal between two Rulers, relying on the acceptance of the target Ruler and a secondary intercessor." 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.