World slots are their own Minecraft server instances. You get full command access to code anything you want, plus WorldEdit and Axiom (for Full Moose rank and above).
They're priced higher than typical plot server plots because each world requires spinning up a separate server instance when it's active. Since everyone gets full command access, we can't run multiple people's worlds on the same server.
Every world slot purchase helps keep the network and everyone's worlds online!
(All ranks also include bonus world slots, with higher ranks including more.)