Each ship in the Merchant Fleet serves under an arrangement called a merchant charter. The different types of charter all take the form of a feudal oath sworn to the fleet authorities on behalf of the Emperor.
Heredity Free Charter
The Heredity Free Charter is the most coveted and highly honoured form of captaincy. A hereditary free captain nominates his successor, and that successor swears the oath of allegiance and thereby becomes the new captain of the ship when its current captain dies or retires.
The captain is "free" in that he may trade freely within the Segmentum where his fleet is based. Many of these old captaincies are based in all five of the Segmentae Majoris. Although the hereditary free captain is theoretically an Imperial servant, his obligations are few. The ship may trade where and how it pleases within the confines of its charter.
Hereditary captains may pass their ships to favoured, or related, successors as described above. As well as inheriting a ship, the captain inherits a route or routes, and can only carry cargo and passengers along this route. Some routes are more profitable than others and so are more highly regarded.
Free captains are appointed to command individual vessels by fleet officials. They are usually established fleet officials themselves, having worked their way up the ranks to a position of responsibility. Free captains may trade as they wish within the Segmentum, except that they are usually forbidden from trading along established routes. Instead, they roam the less-densley populated sectors, areas where regular services are either not needed or would be too costly to run.
A fleet captain is appointed to his position in exactly the same way as a free captain, but plies fixed routes like the hereditary captain. This is the least prestigious level of interstellar captaincy, and is also the least secure. A fleet captain may be deprived of his command and given a shore posting at anytime, his ship reassigned to someone else.