A Starfleet department charged with investigating its own personnel, whose name Luther Sloan borrowed when he interrogated Doctor Bashir on behalf of Section 31.
The Department of Internal Affairs is the part of Starfleet responsible for looking into suspected wrongdoing and breaches of security by officers. It answers to Starfleet Command.
It figures in Inquisition. Sloan arrives on Deep Space Nine introducing himself as the department's deputy director, confines the senior staff to quarters and questions Bashir about the weeks he spent in a Dominion prison, arguing that he is an unwitting spy.
The whole inquiry proves to be a holographic test. Sloan really belongs to Section 31 and has been judging whether Bashir is fit to be recruited. When Sisko afterwards checks with Starfleet, he is told that Internal Affairs has no record of any deputy director called Sloan.