From Silver Sun Republic Codex

The true social elite of the Republic are the governing caste of the Nobilitas. While most are self made there are others who have risen from immigrants all the way up to Nobilitas. Very few true blooded Nobilitas are alive today and there are some who maintain the blood, but have been banished or exiled being prevented from proclaiming their true rights of status.

Nobilitas may achieve an even greater status having been bestowed a Great House with a charter from the Republic's Senate. Great Houses allow patronage which means any Plebeians or Eques may join and adopt the family name. Not all who join Great Houses immediately or ever use the house name, but they wear the colors of the house and are loyal to the Matriarch or Patriarch who provides them patronage. Each Great House has its own set of colors, preferred weapons, magic, Ministry officials, deities and many other elements which make them unique and united.