A primary space is comprised of one or more sub-spaces (also called areas). A single project can have multiple primary spaces and each primary space can have multiple sub-spaces.
Consider a ballroom for example. A ballroom may be divided into multiple smaller spaces for multipurpose use. The ballroom would be considered the primary space and the division into smaller spaces would be the sub-spaces.
Primary Spaces may be created either in the New Project Wizard or from the Browser in an existing project.