The most senior rank in the MetropolitanPolice, excepting only the PoliceCommissioner. The AC's are essentially what they sound like: assistants to the Commissioner, in charge of particular types or areas of operation.
The first two AC positions were created in 1856 under the Police Act; the third in 1884, and altered in 1887.
AC A: internal discipline, administration of the uniformed force
AC B: executive business, supplies, buildings, etc.