From the game's encyclopedia
You can only interact with characters who are within Diplomatic Range. However, neighboring Realms are always in range.
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.
Diplomatic Range is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "You can only interact with characters who are within Diplomatic Range." The game also links it as Range. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
for a guide, post or blog
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.