"SS "stands for Screwdriven Steamship, whereas "RMS" stands for Royal Mail Ship or Royal Merchant Ship. A steamship would normally be referred to as "SS" unless it was under contract to carry mail, when it would use the prefix "RMS" ( Royal Mail Ship). Therefore Titanic is correctly referred to as "SS Titanic", and also "RMS Titanic" as she was carrying mail under the auspices of His Majesty's postal service. When not carrying mail, a ship would revert back to "SS".
All ships, military and civilian, that were under the British flag carried the distinction of "R.M.S." This, in effect, gave the ship the protection of the British Crown. An attack on an R.M.S. was considered an attack on the crown and an act of war.