The Imperium is a the collection of hundreds of worlds lead by Emperor Karr. The worlds are informally divided into two major groups: Core and Mantle.
The Core worlds conspire of roughly 20 of the oldest, most powerful worlds closest to Nexus.
Mantle worlds are all the remaining worlds in the Imperium. Mantles worlds were founded after the Great Expansion almost 500 years ago. Mantle worlds are much less developed than Core worlds not just because they are younger, but also because far many more Mantle worlds are colonized in a shorter amount of time.
Prime worlds is another term that citizens of the Imperium often use. It is used to designate worlds with conditions similar to or better than the homeworld of Nexus. Prime worlds can be found anywhere and, in fact, are much more common in the Mantle.