The change is that you can pick to make a claim for a county or the duchy if it's a duke/duchess when the plot succeeds.
The term Casus Belli literally just means 'reason to go to war.' It's the justification for raising troops to try to conquer or kill an enemy. I managed to get a cassus-belli by marrying the Doge of Pisa, having a son by him, and then assassinating him after he dared to sire a bastard.
Crusader Kings 3 Casus Belli is a video how to get claims.
