Planes of Existence
In the Terracopia setting, access to other planes of existence is not readily available. There are only a handful of known extra-dimensional spaces and access to them is done through portals or gates, always. Terracopia does not have a concept of inner or outer planes, nor does it have an astral, ethereal, or shadow plane.
Terracopia does not have concepts of Angels, Devils, Demons, or any other extra-planar species or creature. There are no aligned planes.
Elemental magic is drawn from the power of the real world itself. Elemental spirits move freely in the "prime" material plane.
Some cultures have developed magicks that allow them access to extimate spaces. These are similar to demi-planes but are closer to the idea of folded space (bigger on the inside). Objects such as bags of holding or portable holes access extimate spaces, as do spells like Rope Trick. Extimate spaces cannot co-exist: You cannot put a bag of holding inside of another bag of holding.
Religion and the Planes
Cultures that believe in deities may also believe that those deities inhabit other planes of existence. Each religion will have its own beliefs regarding this, or mechanism of transport between the areas. Whether or not these "planes" actually exist is an argument for philosophers. No living creature has ever reported travelling to them and no one who has returned from death remembers being "dead".
Many of the gods in these religions (such as Haephus, Dagen, and Pazuzu) make their homes in the "prime" world of Terracopia, though the locations of their palaces are rumors at best. Haephus is said to sleep beneath a great volcano, Dagen makes his home atop the tallest mountain, and Pazuzu lives in a castle among the clouds.
The Court of Heaven
This is the place where souls are believed to go after death. It is said to be ruled over by the god Ba'al, and is home to the fourth Celestial Tree. It is sometimes called the "Grey Lands" or the "Land of Dust".
The Moon and the Great Apparatus
The Great Apparatus is a device that is maintained by the goddess Notfari. It is said that the Great Apparatus exists on the moon, where Notfari resides.
The Corruption Dimension
The Corruption Dimension is a special case. This are could truly be thought of as an alternate plane, according to standard rules. Access to the Corruption Dimension is available only through Blight Gates - portals that exist inside of Blight Zone areas.
The Corruption Dimension has adverse affects upon those who visit it. It is a place of madness where the rules of reality are unwritten and then rewritten. Those who have crossed its threshold return forever changed: many are irrevocably corrupted, and others are driven insane.