The word world is defined by all the space traversable without magic, with its own physical laws and natural order. The Beast World, Ancestral Homeland, and Broken World are all material worlds. Some worlds don’t meet a material world’s qualifications, but are still discrete spaces with their own rules.
“Plane” and “world” are interchangeable. “Plane” more often describes a pocket world in the Astral Sea created by a magic user, while “world” is more commonly used to describe naturally occurring infinite places like the Beast World.
The most fully realized and complex of the planes are material worlds. Unsolved material worlds are the only ones capable of nurturing willful creatures, and the three sources of magic are all represented at some point in any material world’s lifespan. Rebirth and reinvention are constant in the material worlds. The symbiosis of Divinity, Arcana, and Nature allows willful creatures to shape their destiny. They shimmer in the Astral Sea as a precious coalescence of unknowable powers. Some believe material worlds to be the purpose of existence itself. In a solved world, transformative decisions are no longer made by its willful creatures. These material worlds have settled into a final state. Divinity languishes from malnourishment in a solved world. Without the conflict and resolution that sustains a deity’s portfolio, they eventually leave to observe a new world. When that happens, the balance of fundamental forces is indefinitely disrupted. Native willful creatures lose the ability to procreate, as Nature won’t manifest willful young who go unobserved by Divinity. Worlds are only solved through the actions of willful creatures, and it’s usually by accident. A solved world may meet a grisly fate in which willful creatures exterminate themselves completely. Without anyone to behave deliberately, there’s no reason for gods to stay. However, there are other ways a world can reach a solved state. In some worlds, every willful creature is isolated from every other one. Their decisions are never challenged enough to spark the gods’ portfolios. Conflict ends. The world is solved. This was the Ancestral Homeland’s fate. Meddling with fundamental forces is the rarest way to solve a world. A few willful creatures can disrupt the synergy that sustains their world and send it into a death spiral. The result is a desolate existence whose maimed Nature undergoes a cascading failure to enforce its own rules. This is believed to be what began the Broken World’s unraveling. Some worlds are byproducts of others. These manifested worlds are often tiny, closet-sized planes created in the Astral Sea by cunning magicians. Larger manifested worlds are the handiwork of gods. The Dreaming is such a world—its space is manifested by thoughts gleaned from sleeping beasts and used as a resting place for the Seelie. Manifested worlds can also be created by natural side effects. The Netherworld was created by a quirk of existence and is outside any power’s direct influence. The Netherworld is coterminous with the Beast World. This means there’s a 1-to-1 relationship between space in the Beast World and the Netherworld. There is nowhere in the Beast World without a Netherworld counterpart, and vice versa. This makes the Netherworld useful for hiding or moving through solid objects, but using it to teleport is impossible.