The feudal system from both the Middle Ages and Crusader Kings III revolves around Vassals, characters who are granted by a Liege titles to more easily manage the realm in exchange for a cut of their taxes and levies. Each Vassal can be a Liege towards its own set of Vassals as long as lower titles exist.
The amount of taxes and levies granted by a Vassal to their Liege depends on their Opinion towards the said Liege.
In each realm the Vassals with the highest tax income and number of levies are designated as Powerful Vassals, Vassals who expect to be part of the Liege's Council and will have vastly lowered Opinion of their Liege if they're not. The number of Powerful Vassals a Liege will have depends on the rank of their primary title. In order for the succession law of a realm to be changed all Powerful Vassals must agree. In elective succession types Powerful Vassals have more votes than regular Vassals. They're also much harder to use the Sway Scheme on.