Acting ( rank ) 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 . Addressing . 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 seaman would be addressed as " Master Seaman 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 . " Other ranks are usually referred to in written documents as " Sergeant ( Acting ) " or " Captain ( Acting ) " United States Navy . In the United States Navy , acting appointments were common during the 19th century . The number of commissioned naval officers at each rank in the Navy was fixed by Congress , so it was difficult to fill vacancies if the number of officers needed to man ships exceeded that fixed number of officers allowed by Congress . Acting appointments were also common with warrant officers and ratings , although neither were subject to congressional approval and were simply temporary assignments . The regulations stated that in the United States , 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 States and not part of a fleet or squadron , the commanding officer of the ship was allowed to appoint officers to a higher rank in the case of death on board the ship . 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 . Another type of temporary appointment was an " order to perform " . This was issued in a similar manner to an acting appointment for a lower grade officer to perform the duties of a higher grade officer , except that their pay , rank and uniform remained at the lower grade . Similar to the many brevet ranks in the Union Army , acting appointments were extremely common during the American Civil War . Congress authorized the Department of the Navy to purchase vessels and appoint acting or volunteer officers to man them until the end of the conflict . By the end of the War , most officers were appointed to a higher acting rank , and their appointments lasted until the end of the war at which point many were discharged from the Navy .