Beyond the 15 existential planes, 5 parallel dimensions exist — complete universes with their own physics, their own gods, and their own versions of everything. Travel between them is possible but catastrophic.
Planes are layers of the same universe — like floors in a building. Dimensions are entirely separate buildings. Traveling between planes is difficult. Traveling between dimensions is an act of cosmic violence that tears holes in both realities. Every dimension-crossing leaves scars that take centuries to heal.