Pathfinder Player Companion: Plane-Hopper’s Handbook - Rules of Play says:
Step beyond reality as we know it! Pathfinder Player Companion: Plane-Hopper's Handbook offers new tools for explorers brave enough to venture into the unknown—whether to abandoned cities created by dead gods, worlds where the ground is made of forsaken souls, or even stranger realms. Add planar allies, magic, or heritage to your adventures and infuse them with the power of the planes!