Vassalage

Vassalage is the practice of one House swearing fealty to another, providing economic and military support to its benefactor in exchange for protection and the redress of grievances. The subservient House is a vassal of the leading House. There is a limit to how many vassals a leading House can manage. A master can depose a vassal, but this may risk rebellion.

Codex
"The relationship between houses in the political structure of the colonies is that of vassalage, i.e. a system of mutual obligation between the master house and the subject house. The primary obligation of the master house is to provide protection and justice to the subject house, while the primary obligation of the subject house is to provide military and economic support.

In a faction, a leading house has the ruling houses of its colonies as vassals. Each ruling house pays a tax to the leading house, and must answer a call to arms and provide their military fleet for the leading house's military adventures.

The ruling house in turn has its own vassals; the noble houses that populate the colony. Thus when a leader goes to war, their fleet is composed of the combined ships of all the houses on all the colonies of their faction.

Vassals enjoy considerable autonomy, largely because of the distances between colonies and other installations. As a result, even powerful houses have limited tools to directly control their subjects, and holding together a large faction or league can be more challenging than conquering it.

Independent houses are houses that are not the vassal of another house. All great houses are independent. Some leading houses are independent, while some are the vassals of a great house.

A leading house becomes a great house by having other leading houses as its vassals. Note that this can only happen when it is independent."