In the Ex Dynamis Chaos EDK universe, subspaces are considered pockets of alternate spaces hidden within the biological matrix that is everything. The concept of subspace is first explored in Ex Dynamis Chaos episode 3: Night Games when The Metal and Buck travel within what the former refered to as a transit, or subspace highway, that led from Oak Cliff in Dallas, Texas to presumably somewhere in Norway.
The Metal refers to them as transits most off, but everyone knows them as subspace highways. Subspace highways are a form of subspace that connect two or more parts of the universe together, be them near or distant, in way that falls in line with wormholes. Not all subspace highways are the same and their structures don't have to follow suit with the rest of the universe since they are pockets that are separate but within. This is noticed in subspace's first appearance in EDK as Buck doesn't even notice the entrance of the highway or that he was in one until The Metal points it out. It appeared to be an alley without floors or ceilings, but a fixed plane and gravity were still present for them to walk upon.