Rogue Trader is the possessor of the sacred Warrant of Trade. The Warrant of Trade is a reward given to an individual or a family dynasty as a reward for exemplary service to the God-Emperor. The Warrant allows the Rogue Trader to own and control one of the great star ships that are capable of Warp travel. Within the Imperium of Man, a Rogue Trader is seen as someone with the same rights and powers as a planetary governor or an Imperial Navy Admiral though they are still technically bound by the same laws that govern all men.
The true power of the Warrant of Trade is that it grants permission for the Rogue Trader to travel beyond the Imperium of Man to expand humanity’s domain. Whether this is done by colonizing new worlds, establishing trade routes with xenos races, or rediscovering lost human races settled before the Time of Troubles is all up to the Rogue Trader’s whim. Beyond the Imperium of Man a Rogue Trader is held by no law and his rule is absolute.