Core Space is the region within the Hiberverse tightly controlled by the High Hibernal Council. While Core Space is continually expanding as the Hiberverse pushes further outward in its efforts to sequester BDM, Fringe Space is a constant reality as well.
Fringe Space constitutes the portions of the Hiberverse where High Hibernal Council rule is limited to non-existant.
The Jump Gate system of instantaneous travel also plays an important role in dictating whether space is Core or Fringe. Council rule quickly fails in areas unavailable to the Jump Gate system.