Dark Heresy: Sins of the Father
Each of the Imperium’s worlds have their own government, laws, and their own local police forces to enforce those laws. The Arbites concern themselves only with the enforcement of the Imperial law, the broader laws to which the entire Imperium is subject.
Utterly dedicated and without mercy, the Arbites are feared throughout the galaxy, for they are the agents of a harsh law, where failure and incompetence are crimes, and the only punishment is death. Arbites are empowered to act as judge, jury and executioner – citizens have no rights, only members of the Priesthood of Terra or the Inquisition could claim anything so elaborate as a trial.