A star system consists of one or more stars plus whatever planets, comets and asteroids orbit it. Composition can vary a lot but for every star system there is a designated mainworld. This is almost always the most populous planet in the system, and will have this status for some good reason. Usually this means good habitability or resources, though there are sometimes other reasons why a world is settled. Where there is no obvious mainworld, one is usually selected based upon its relative position to the system’s primary (star). This might be an airless rockball in some systems, and another world could in theory be designated mainworld if the system is later settled and a different planet is chosen as the main location.
Written by fan-favourite M J Dougherty, A Guide to Star Systems is an invaluable addition to the Referee's Aid series, exploring the different systems of the galaxy and discovering how they can be used to influence campaigns.