An opinion.
I was looking at the game and considering playing when I saw this discussion.
I like theoreical debate, especially when reality is slightly suspended so.....
I was an avid fan of babylon 5 and Star Trek, and I've read almost everything Alan Dean Foster and Larry Niven ever wrote., along with a lot of E.E. "Doc" Smith. My own theory on 'subspace' and 'hyperspace' would be this:
Subspace is any reality that is included in moving through space time wihtout interference or utilizing movement inside space itself, such as wormholes, jump gates, teleportative devices or the like.
Hyperspace includes any type of faster than light travel where the traveler is still within normal spacetime but not necessarily subject to its effects, such as warp-speed, inertialess drive, ion drives, and other FTL movements.
I haven't dealt with realspace, so I couldn't make an opinion.