You probably would not have heard of cornet as a rank as it was likely fairly rare. It was the cavalry equivalent of ensign, and had pretty much the same duties, carrying the troop (i.e., company) colours. I believe they were attended by or even carried the cornet (trumpet) for signals, cavalry being notoriously deaf. At least they were reluctant to give attention to orders. The disastrous charges of the Light Brigades at Waterloo and Balaclava were not exceptions.