Accounting (abbreviated as Acct.) is a section of the Federal Bureau of Investigation that is based at the FBI Academy in Quantico, Virginia. (MM: "The Sound of Snow"; TXF: "Alone", "Scary Monsters")
In 1999, a standard internal mail was sent within the FBI Academy and contained a list of names, one of which was Jeff Gore who was said to work for Accounting. (MM: "The Sound of Snow")
Agent Leyla Harrison also worked in Accounting and, therein, processed travel expenses of Special Agents Mulder and Scully, gaining an insightful knowledge of their work on the X-Files. After she begged to be assigned to the X-Files unit herself and her request was approved in 2001, Agent Harrison returned to Accounting following her work on only one X-file, partnered with Special Agent John Doggett. (TXF: "Alone", "Scary Monsters") In 2002, she remarked to Agent Scully that, even though she had relished her big adventure on the X-Files unit, she was happily back in Accounting with no regrets. (TXF: "Scary Monsters")