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An assistant chief medical officer is a senior title held in the Federation Starfleet's medical department and whose authority is second-only to the chief medical officer. The ACMO is tasked with assisting the CMO in medical matters, and is trained to take control of the medical department should the CMO be absent.
Prior to 2265, Leonard McCoy had served as ACMO on a starship before his assignment to Starfleet Medical. (TOS - My Brother's Keeper novel: Constitution)