From the game's encyclopedia
A Bloc Expansion War is a special type of War waged by a Bloc member.
If victorious, the winner may appoint a Ruler to the contested Titles, or retain them for themselves.
Appointing a Ruler in this way will add a Hook on the recipient, add them to the winner's Bloc, and found a new House if the recipient is not a House Head.
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.
Bloc Expansion War is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Bloc Expansion War is a special type of War waged by a Bloc member." It is part of House Bloc. The game also links it as Bloc Expansion. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.