Technically, all passenger liners required life support, escape pods, a qualified crew and a valid license, supported by the appropriate insurance and government dues. In practice, the term denoted any ship that could carry passengers besides its flight crew. The two main types of passenger liner were the small passenger ship (SSP, for "small starship, passenger"), denoting ships under 100 meters long with less than 500 passengers, and the luxury liner, over 100 meters and with more than 250 passengers. Implicitly, ships in the two size brackets with passenger complements outside these ranges probably had different designations, but such ships were probably much less widespread.
Passenger liners have occasionally been used in military operations. Most notably, the Alliance to Restore the Republic converted a number of passenger liners into warships, including some Mon Calamari Star Cruisers and the Sullustan design that became the Dauntless-class heavy cruiser. Also, Palpatine's personal luxury liner, the Excalibur, saw battle in a coup attempt by Grand Admiral Demetrius Zaarin, and the SSP designation extended to many freighters and shuttles that were capable of basic combat.