Planets are classified by degrees of population and wheter or not they do support or oppose life in general.
Prime Worlds & Capitol Worlds
These worlds are sually but not necessarily the most populated planets within a star system. They are are considered the political and coltural heart within a system. As capitol worlds they are usually the seat of government and all major institutions. While not all systems do have a prime world, those who do are sometimes (re)named after this world rather than their primary star.
Secondary worlds classify habitable planets, which host a measurable population without holding prime or capitol status. Their population varies from small settlements to bustling metropoles rivalling those on prime worlds.
Tertiary Worlds classify planets that are mostly uninhabited, but do support sentient life in some degree. These worlds usually are host to industrial faclities such as mining outposts or manufacturing plants, as well as military outposts, all of which may be located anywhere on the planet's surface or within its orbit.
These planets are generally flagged as uninhabitable due to adverse conditions that impede or even threaten life, both on or off-planet. Therefore civilian and commercial travel to these off-limit planets is often prohibited for safety reasons. They may yet harbor military installations or guard posts. Convention has it that off-worlds usually receive no name but are rather designated after the system they are located in, for example Thalas III.