Race

In the Tensei Shitara Slime Datta Ken universe, all races are classified into two basic categories, physical and spiritual beings, each of which are further divided into three more sections: Humans, Demi-Humans and Monsters for physical existences, and Angels, Spirits and Demons for spiritual existences.

The Soul and the Origin of the Major Races
Sister article: Soul

In the Tensei Shitara Slime Datta Ken Universe, all living beings have souls. If something doesn't have a soul, it isn't considered a living being at all. Meaning, there can be some creatures that live on pure intention or instinct but have no soul.

All souls/living beings have three layers: the astral body, the spiritual body and the physical. The Core/Soul is the being or individual itself. To be considered a proper existence, the soul needs an astral body. This acts as a container for the individual's existence. However, with only an astral body, the soul will only dissipate into pure energy again, hence a medium to record memories is needed, which is the spiritual body. These memories keeps the existence of 'self' intact, but even so, it is not permanent, which is why the physical body is needed.

The final rule only applies to souls that wish to reside in the physical world. Veldanava created two worlds, the spiritual and the physical, neither of which can freely interfere with one another. For a spirit or soul to reside in the physical world it would need a physical body. And for a physical being to survive in the spiritual world, it would need to be able to transform into a spiritual lifeform.

Thus the two worlds were kept separated from each other, aside from some exceptions such as True Dragons and beings with high magical quantity, who can create a physical body using there enormous magic reserves, and Demon Lords and Demi Gods who have become spiritual existences.

Till now, there were simply the physical and the spiritual, and nothing more. But later, Veldanava, who had been observing multiple worlds/universes falling into chaos and war brought about another change, which was 'alignments'. He noticed that among the spirits, some were inherently 'bad' and others inherently 'good'. And among the physical world, there were always 'humans', hence a gradient of humans to demi-humans to least human (monsters).

And among those beings, he noticed, were individuals with more matured souls. These individuals were later made the overseers of the three alignments of spiritual beings. Dino, the leader of the orderly and 'good' aligned race, the Angels. Ramiris, the leader of the free-spirited and 'neutral' aligned race, the Spirits. Guy, the leader of the chaotic and 'evil' aligned race, the Demons.

Terminologies
It is to be noted that all beings, regardless of physicality or spirituality, with some form of "evil" alignment are referred to as "Demonic-kin" 「魔族, Mazoku」. This can be especially confusing in translations as Mazoku is written as both 'demons' and 'monsters' in different circumstances. Mazoku includes all of Monsters, Demons and Devils (魔 meaning "evil" or "demonic").

Next are Monsters 「魔物, Mamono/Mabutsu meaning "Demonic Beings/Creatures"」. They include all physical beings that don't have a 'proper civilization' and don't have many, if not any, relations/treaties with civilized races. Examples are Goblins, Orcs, Fanged Wolves, Slimes, Black Serpent, Giant Spider, Lizardmen and many others.

Devils 「魔人, Majin meaning "Demonic People"」 refer to any humanoid Mazoku. This includes Beastmen, Ogres, Kijin, Harpies, Dragonoids and individuals like Gelmudo, Clayman and Laplace. Sometimes, depending on their reputation and power, certain Demi-Humans may also be referred to as a Devil. Also, Demons that take on a human or humanoid appearance when summoned can also be called a Devil.

Finally Demons 「悪魔 Akuma」 are very specific. They only refer to the spiritual beings known as Demons. They are a race of their own and have their own evolution chain.