The initialism was originally a bit of shipbuilder marketing—and now there are many different kinds of ship designations.

The Titanic, which sank in the North Atlantic on April 15, 1912, had two sister ships almost identical in size and luxury: ...

Titanic's full title was RMS Titanic as it was a Royal Mail ship carrying nearly 3500 sacks of mail including all sorts of letters and packages. Titanic's construction cost was £1.5 million ...