Subspace is defined as the set of all realms that do not contain themselves.
Subspace Highways
Subspace connects realms that do contain themselves through subspace portals. All subspace realms connect to non-subspace realms on at least 2 ends, forming subspace highways. Due to the unusual nature of subspace, no life can exist in subspace. Subspace can be traversed with spatial stabilization technology or warding magic.
Metaspace Highways
Metaspace highways connect two points in space that are within the same realm and domain. Metaspace highways can be traversed by conventional methods of locomotion, and are not hostile to life.