An acting rank is a designation that allows a soldier to assume a military rank—usually higher and usually temporary.
Acting appointments were also common with warrant officers and ratings, although neither were subject to congressional approval and were simply temporary assignments.
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.
[1] 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.