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human
[Warhammer 40K] The Inquisition Vs The Adeptus Arbites

It seems to me that the roles of these two organizations overlap somewhat, with perhaps the deciding difference being accountability — Arbites do actually follow laws and codes and consider things like legal precedent and whatnot in their actions.

Despite their reputation, the Arbites also has investigative ranks, such as detectives who investigate possible crimes as well as chasteners who extract information from suspects.

The Inquisition focuses most of its efforts inward, effectively trying to identify and destroy covert incursions that obviously already run afoul of Imperial law.

For instance, the black market trade of xenos artefacts will attract the ire of the Ordo Xenos (or Ordo Hereticus) just as it would the Arbites.

As well, at the highest level, a Judge of the Arbites holds nearly the same powers as an Inquisitor.

So why are there two organizations?
gpt
Arbites are law enforcement, their duty is vigilance in ensuring life goes according to Imperial Creed. The job of Inquisitors is to deal with the enemies of the Imperium in a capacity beyond standard warfare. They root out cults, exorcise daemons, manipulate xenos, on a level far more complex and far reaching than the arbites.