I use S for small, which means something less than 800 km in diameter, it is a general catchall. Besides Pluto is not a planet anymore, so I can't list it in good conscious without listing Sedna, Xena, and others, and it appears their is a whole belt of these objects, so I decided to stop at Neptune. Phobos and Demios are important to terrestrial worlds such as Mars and Earth which don't have many Moons, to gas giants such small potato shaped moons are a dime a dozen, but if their is an 8 mile wide something orbiting an Earth-sized world, I think it is important.