The term small passenger ship (or SSP, for "small starship, passenger") was the general designation for passenger liners less than 100 meters long, carrying up to 500 passengers.
The precise definition of the classification varied somewhat in different sectors, but apart from the basic constriants of size and capacity, the only limit that the Galactic Empire placed on the SSP classification was that a ship had to operate under a license from the governor on the specific planet where it claimed its home port. The planetary administration was responsible for ensuring that the ship satisfied Imperial safety regulations, and also for levying appropriate fees and dues from it.
Some planets were known for offering cheap licenses and lax regulations—and some SSPs were known to hold licenses from multiple home ports: by presenting a license from another port, the ship's captain could permanently avoid fulfilling his responsibilities.
Due to the basic limits on the designation, the SSP category included almost every small ship with a passenger license, whether it was a shuttle, freighter, or star cruiser. They were, however, divided between "line-operated" ships and "independents." Lines typically provided regular inter-system services within a single sector; independents included every individual captain whose ship was licensed to carry passengers, with all the variation that implies—though they rarely pursued a fixed schedule of routes.