Worlds and Realms

"Think of Realms like islands in the sea, except every island is also touching several other islands at all times. And probably has its own islands inside itself and the distance between islands is changing all the time... you know what, forget what i just said."

— Miriam, Realm traveler

A Realm is a dimensional location within the multidimensional universe of the Brass Realms. A Realm is best described as its own, self-contained world. How this world looks or functions can vary greatly.

Material Realms

The "Material Realms" or "Mortal Realms" Make up one part of the Brass Realms. Material Realms are self-contained worlds governed by immutable laws of physics and causality, with each Realm being a sort of bubble that floats in the Astral Sea. Realms are either naturally occurring or they are pocket-dimensions that are artificially created and can be reached through magic.

Types of Worlds: Realms have 'Types'; broad generalizations that are used as short-hands by Realm travelers and explorers. Examples might be describing a world as an "Ice world" for its generally low temperatures or a "civilized world" if it is inhabited by a powerful sentient species.

Realm Travel: Realm travel generally occurs by means of Realm-gates, Nexus Keys or some other means of portal-creating magic to allow a mortal to step from one material realm into another. This is by no means common or easy but it is not a secret either.

Realm Shell

A material realm is separated from the Astral Sea by a Realm Shell. This is the invisible barrier between the Star-Sea and the realm in question. The Realm Shell is formed out of Arcane energy that is sourced from the Divine Machine and is critically important in sustaining the realm's laws of reality.

Realm clouds:

Realms appear in the Astral Sea in the form of 'Realm clouds'. These worlds are not openly accessible or even clearly visible. Instead, where the worlds float in the astral they cause a faint gaseous outline of the world to appear which shares some traits with the world it belongs to.
If portal-creating magics are used on these clouds, it is possible to create temporary gateways to cross into a world. If one uses similar magic inside a world, it will create a gateway in the astral instead.
The locations of these clouds are highly valuable information for navigators and astral sailors.

The Spirit World

The Material worlds are not all there is to the brass realms. There is an entire second set of realms, hidden away from mortal eyes: the Spirit World and its associated spiritual realms.

Similarly with how all material realms float in the astral sea, the spiritual realms instead float in the spirit world: a world of inscrutable mist and calm stillness. However that is where the similarities between material and spiritual realms end.
Spirit realms are beyond the real, they deal not in physics but metaphysics, same as the inhabitants that originate from them. Be it the Seven Heavens or the Nine hells, exploring these realms is not done lightly.
The inhabitants of spiritual realms are also not mortal creatures or monsters but instead immortal spirits; beings so strange and alien that it is hard to believe they were mortal once as well. Perhaps they weren't.

Visiting the spirit world is not generally possible while still alive, and neither is it possible to take anything along for the journey. The journey into and through these places is a journey of the mind and soul, not of the body. Certain mystical procedures exist to allow mortals to visit the spirit world temporarily, however.
A wandering soul or visiting mortal mind must then find their target destination by wandering the mists of the spirit worlds and attempting to reach them.
The Locations of these worlds as well as what is within them is fluid and depends more on the visitor and their state of mind than anything concrete, objective and constant. The nine hells for example are pits of vile evil independent of what cultural norms one has, for they contain them all.