ACO was a department, division, or position within Starfleet's Fleet Operations Center on Earth. (ENT: "Demons" set artwork)
In January of 2155, Starfleet officers J. Nesterowicz and P. McWade worked in (or as) ACO.
ACO was seen on directory listings seen on the set of "Demons".
, like many of the other divisions, departments, or positions on the listing, is a reference to NASA
terminology, in this case, the "Assembly and Checkout Officer".