Of the nine worlds in the Altris Crystal Sphere, only five are able to sustain life. There is no real relationship between these worlds, though there are merchants and traders that do frequent many of them.
Altris is the most diverse holding a multitude of races and cultures, whereas planets like Zoln and Kalamire hold only one race and generally only a single culture. Froltoil, even though it can support humanoid life in some areas, is vastly uninhabited and there are no permanent settlements on the planet.
The additonal planets that can not support humanoid life are; Cani, Drawmir, Humphet and Solic. All but Cani have atmosphere, however these atmospheres are toxic to human and demi-humans. An explorer would require some type of magic or device to allow them to breath. Solic's atmosphere is acidic and will burn the skin and eat through leather and steel - not to mention what the acid will do to the lungs. Cani is nearly a ball of multen rock with perhaps a few shifting islands of solid ground that can be found at night, but when the sun rises, these masses will melt away.