From the PDF Introduction
Variant Planes in Your Campaigns: A Manual of the Planes Web Enhancement
The new Manual of the Planes book isn’t just a toybox filled with cool completed toys. It’s also a toolbox you can use to create your own planes. Like the earlier edition, this one goes into great detail on the core cosmology of the D&D® game universe. Unlike the previous version, however, this book also includes a number of variant planes that you can use in addition to (or instead of ) the Core Planes. In this way, you can create a unique fantasy universe of your own, molded to fit
the needs of your campaign. Your universe need not be the same as another DM’s, any more than two dungeons in your campaign world need to be the same, though they may use the same components.
The various planes are categorized into five general groups. The Material Planes are the various campaign worlds in which characters may adventure. The Outer Planes are the homes of gods and other great powers, as well as the "final resting places" for spirits of the dead. The Inner Planes are wellsprings of elemental forces and pure energy-the building blocks of the cosmology. Demiplanes are finite planes with limited access, often created by magical force and mortal endeavors. Stitching all these together are the Transitive Planes, which allow movement along and often between other planes. In the core cosmology, the Astral Plane, the Ethereal Plane, and the Plane of Shadow are all Transitive Planes. The World Serpent Inn (described in detail below) is also a Transitive Plane. You can use it in addition to the core Transitive Planes, or as a replacement for one of them.