Fantastic Ship Prefix: Several. The Empire alone uses CS (“Coronals’ Ship”) for Imperial Navy vessels, CSS (“Coronals’ Service Ship”) for governance vessels that aren’t part of the Imperial Navy, CMS (“Coronals’ Merchant Ship”) for commercial vessels, IS (plain old “Imperial Ship”) for private vessels other than commercial…
…and that’s before other polities start getting in on the act.
(Hull numbers are usually a three-part compound: ordering organization, ship type, and procurement number – say IN-BC-4129 for CS Machyphage – but are rarely used for anything but database keys.)