Arbitrator-Auditor is the official title of the person appointed by the Imperium to go through a planet's tithe-books. It is the Adeptus Administratum that actually collects and counts the tithe, but the Auditor is always from the Adeptus Arbites, since they are the only people trained in the finer points of tithe law. The Arbitrator-Auditor is always a senior judge and attended by a sizable staff (exactly how big depends on the size of the tithe). Being appointed Auditor is a great honor (and usually a lot of work).