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.
Domain 1
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.