For example from the records of the 16th New York:
AUSTIN, JOHN H.-Age, 26 years. Enlisted, May 6, 1861, at De Peyster; mustered in as private, Co. G, Mary 15, 1861, to serve two years; promoted first sergeant, date not stated; mustered in as second lieutenant, October 21, 1862; mustered out with company, May 22, 1863, at Albany, N.Y.; commissioned second lieutenant, December 18, 1862, with rank from October 21, 1862, vice Bayne promoted.
In this case, Bayne refers to Andrew C. Bayne who was promoted to first lieutenant in place of William L. Best who was also promoted.
Ryan