The Merchant Fleets of the Imperium of Man are the commercial component of what is collectively known as the Imperial Fleet, which comprises all known human spacecraft in the galaxy loyal to the Imperium and also includes the Imperial Civil Fleet and the Imperial Navy's Segmentum and sector battlefleets. The commercial spacecraft of the Imperium which comprise the Merchant Fleet, most of which are commanded by the Chartist Captains, make up approximately 90% of all the spacecraft with interstellar capabilities in human-controlled space.
Each of the Merchant Fleets, much like the Imperial Navy's battlefleets, are based in one of the five Segmentae Majoris that comprise the primary administrative units of the Imperium and their staffs are based at the respective Segmentum Fortresses. For example, the Segmentum Solar fleet is based on Mars, whilst the fleet of the northern zone -- Segmentum Obscurus -- is based on Cypra Mundi.
While these fleet bases are massive orbiting void stations that usually maintain gigantic complexes of orbiting docks, shipyards and starship repair facilities, their primary function is to maintain bureaucratic control over both the military and civilian starships operating within their Segmentum of control. In reality, of course, only a vanishing fraction of the starships assigned to a given Segmentum's merchant fleet ever actually travels to its Segmentum Fortress.
To lawfully carry out trade, a merchant's vessel must obtain an Imperial Merchant Charter from the Adeptus Administratum that allows it to join a Segmentum's merchant fleet. Not all charters offer their captains the same terms, as some offer more powers and more trade concessions to particularly favoured captains, but all are feudal contracts that require a merchant captain to swear an oath to obey the Imperial Fleet authorities of the Segmentum he or she is operating within on behalf of the Emperor of Mankind.
A merchant captain cannot register his vessel with the Imperium or receive the legal right to operate as granted by a Merchant Charter until he has sworn this oath before the fleet authorities of his Segmentum and a record of it has been placed in the records of the Segmentum Fortress of his chosen region of the galaxy and in the Segmentum Solar's Segmentum Fortress in orbit of Mars, the central repository for the Administratum's fleet records.
The most powerful captains of the Merchant Fleets, known as Chartist Captains since they hold an Imperial Merchant Charter, have gained enough political power at certain times in Imperial history to take a seat in the Senatorum Imperialis of the High Lords of Terra. When this happens, this particular mercantile High Lord is known as the Speaker for the Chartist Captains.
It should be noted that Chartist Captains are not the same as Rogue Traders, who are actually far more powerful individuals who have also been granted or inherited a Warrant of Trade that allows them free rein to explore or exploit areas on the very fringe of Imperial-controlled space. Rogue Traders are considered Lords of the Imperium and possess a political rank, if not financial wealth, far in excess of the average Chartist Captain of a merchant ship.
- Rogue Trader: Core Rulebook (RPG), pg. 315
- White Dwarf 139, "Interstellar Travel"
- White Dwarf 140, "Interstellar Travel"