Royal Mail Ship (sometimes Steam-ship, Steamer), usually seen in its abbreviated form RMS, is the ship prefix used for seagoing vessels that carry mail under contract by Royal Mail. They have the right to fly the pennant of the Royal Mail when sailing. Titanic was one of them....
M/S and M/V
A motor ship or motor vessel is a ship propelled by an engine, usually a diesel engine. The name of motor ships are often prefixed with MS, M/S, MV or M/V.