They police the CO's and elder bodies within a district. I know this because a friend of mine was on a body several years back and he told me that they had numerous problems...internal fighting, backbiting, slandering, you know the usual elder stuff but this one got out of hand.
The CO tried to deal with it without success so letters were written back and forth to the WTS he said and the end result was the entire body, along with the CO would go before the DO. He said people were removed, some moved, other elders were sent in. Apparently this type of stuff happens more than publishers realize.