Heaven is the lair of the Chief God and her subordinate divinities, valkyries, and angels. It does not seem to function as an afterlife for the benefit of departed souls, but rather as the private dimension of the celestial gods. Impervious to attack by mortals or monsters, Heaven shares many of the attributes of "Spirit Realms" as described in the Demon Realm Traveller's Guide.
Although it is not explicitly remarked, Heaven is described as the dwelling place of multiple greater divinities, so it seems possible that "neutral" gods like Ares and Eros do reside there. Conversely, corrupted divinities like the Fallen God and Poseidon are categorically banished from the realm.
The world ruled by the divinities. It is said to exist far above the sky, and it's supposed to be a wondrous and beautiful place. In front of the palaces where the gods who are the greater divinities live, the lesser divinities who are servants of a particular god have castle towns, and they number as many as the gods themselves. Only divinities dwell within the realm of heaven, and since only the divinities themselves know the means of traveling between the realm of heaven and the human world, humans and monsters are unable to invade it. However, on rare occasions, lesser divinities who were converted into monsters while visiting the human realm do return there. In that case, it is said that the lesser divinity is banished from the realm of heaven and dropped into the human world.
Divinities (MGE-1, p211)
A general name for the gods, with the Chief God who is said to have created the world at the top. Beings that rule the world and humans from heaven, and spread their own teachings to humans through The Order. They also sometimes give humans instructions directly as oracles. Heaven is a hierarchical society. There's a huge division between the greater divinities, who are the gods, and the lesser divinities, who are angels.
Furthermore, the gods and angels themselves are divided into many different classes. Even the lowest among the lesser divinities who are angels, angel (p. 204), possesses vastly more power than the average human or monster. Speaking of the power of the higher angels and gods, it is even more tremendous.
The realms known as “spirit realms” are peculiar even amongst the Demon Realms. As the name suggests, they are demon realms that exist in another dimension. Most of them were created in new dimensions by gods or monsters with vast power rivaling that of a god. The holy land of the fallen angels that was created by the Fallen God, “Pandemonium”, and the “Wonderland” full of mystery and pleasure created by a certainlilim, etc. are all categorized as “spirit realms”. Since they aren't connected by land, it's normally impossible to enter or leave one of these realms by walking. The method of entry varies depending on each individual spirit realm, but peculiar spells, rituals, portals (teleportation spells), or the assistance of a specific race are necessary.