Domains are a supercategory meant to organize large numbers of levels into groups depending on their properties and how many entrances and exits they share. There is no clear definition for what constitutes a domain, but there is also no clear definition for what constitutes a level. One often given definition is that domains are parts of the Backrooms that are close to each other in extra-dimensional space and that usually share similar properties; but this definition doesn't always work. The distinction between different domains oftentimes has nothing to do with the physical reality of the backrooms: it is simply a tool used by humans to map out the backrooms. Some domains are not really part of the backrooms but are instead completely alternate universes.
Primary Domain
This is the domain that most people think of when they think of the backrooms. It is almost entirely composed of indoor levels; however, some outdoor levels are arguably part of this cluster. The question of whether Domain 1 can be said to include outdoor levels is a matter of much debate in the backrooms. The first nine levels are usually grouped into this domain.