A personnel officer was an officer assigned to the operations division of Starfleet during the 23rd century. In the course of a personnel officer's duties, such an individual would be familiar with the service records of all individuals on a particular assignment.
USS Enterprise had a personnel officer. In 2267, she was called upon to testify on behalf of the prosecution during James T. Kirk's court martial regarding the "death" of Benjamin Finney. (TOS: "Court Martial")