From this introduction to Moroni as “chief captain,” it is clear that the Nephite military organization included sub-captains. Mormon does not say who appointed him, but presumably it would have been the chief judge, based on Moroni’s perceived fitness for the task.
Furthermore, Nephite society has obviously moved another step in the direction of specialization. The day in which Alma, as chief judge, fought in personal combat has evidently passed for the Nephites.