The planes of existence, typically referred to as "Realms" or "Planes", referred to the distinct layers of reality that made up the multiverse. The Material Realm, where mortal life and physical matter existed, formed the reference point from which all other planes were typically described. These realms were connected by Leylines, and each contained Demirealms of various makings. Places outside the Material were generally referred to as "extraplanar", though in truth, the Material encompassed a tiny fraction of all the matter of the multiverse.
Realms were generally grouped into three categories: the Outer Realms such as the Elemental Realms, Upper Realms, and Lower Realms were ever-shifting and more abstract than physical; "transitive" planes served primarily as pathways between other planes, such as the Ethereal Border, Astral Border, Positive Border, and Negative Border; and the places where these concepts combined, such as the Material world and its echoes, the Vivid Realms and Umbral Realms.
Cosmological models of the realms varied by school of thought, and no single system was universally accepted, though the Cosmic Wheel was typically employed to describe planar structure. Scholars generally agreed that the models were theoretical frameworks rather than confirmed structures, subject to revision as new planes were discovered or reinterpreted.
A small number of planes did not fit within standard classification. These existed at the margins of the multiverse, were difficult to reach, and often did not follow the physical or logical rules that governed other planes. Little was documented about them, and most references came from unreliable or secondhand accounts. Creatures originating from these planes tended to be aberrant in form and behavior compared to those found elsewhere in the multiverse.