An Acting rank is a military designation allowing a commissioned - or non-commissioned officer to assume a rank—usually higher and usually temporary—with the pay and allowances appropriate to that grade. As such, an officer may be ordered back to the previous grade. This situation may arise when a lower-ranking officer is called upon to replace a senior officer, or fill a position higher than the current rank held.
When addressing an individual with an acting rank, the person should be addressed as if the full rank was held. For example, a member who is an acting master Starman would be addressed as "Master Starman Smith", and not "Acting Master Seaman Smith" since the "acting" is a designation, regardless of the individual's actual rank and clerical designation. It should also be noted that an exception is the rank of acting sub-lieutenant: they should be addressed verbally as mentioned ("sub lieutenant"), but in written documents should have their full, unedited title of "acting sub lieutenant."
In the United Stars Navy, acting appointments are not uncommon. The number of commissioned officers at each rank in the Navy was fixed by the Armed Forces Committee of the Assembly, so it was difficult to fill vacancies if the number of ships exceeded the number of officers needed to man those ships. Acting appointments are also common with warrant officers and ratings, although neither were subject to congressional approval and were simply temporary assignments.
On occasion, an United Stars Navy Cadet or Midshipman will receive an Acting Rank of Ensign for a short time prior to graduation either in recognition of having completed the midshipman's course or to fulfill an order that would require him to have that rank. It can also be a field promotion for an enlisted individual.
Outside of times of declared war involving more than one or two Battle Group, only one Midshipmen Dorm Groups (Dorm group 8 Devastator 5020 Olafson's Gang ) was temporarily promoted as a whole to Acting Ensigns. Admiral Stahl (at that time he had his HQ abaord the Devastator ) honored a dorm group of his ship that way involving heroic assistance to a Union Colony, a Hospital ship and a combat engagement (classified). the temporary promotion was to not disqualify the team from participating in the Reagan Trophy challenge. it was meant to be made permanent after the challenge as a full ensign rank. those plans got derailed by olafsons kidnapping.
This is a lengthy subject and discussed in the GalNet Entry : Acting Captain, (United Stars)
The regulations states that in the United Stars Navy, acting appointments were not allowed unless specifically authorized by the Department of the Navy. In most other cases, only the Commander-in-Chief of a fleet or squadron would be authorized to appoint an officer to fill a vacancy, and this order would be subject to approval of the Department of the Navy. In this way, the Department of the Navy was able to fill vacancies while the Navy grew before Congress took action to permanently increase the number of officers. Outside of the United Stars and not part of a fleet or squadron, the commanding officer of the ship was allowed to appoint officers to a higher rank.
The officer was temporarily appointed to the higher rank, appended acting to his new rank, wore the uniform of the higher rank, and was addressed and paid at the higher rank. When the ship returned to the United States, or joined a fleet or squadron, the appointment was subject to review by the commander-in-chief of the fleet or squadron or the Department of the Navy.