Each race of demon is classes based on strength. S class being the strongest and D class being the weakest. There have been some reports of demons weaker than D class, but nothing to sustain these claims has been shown. The human world has a natural barrier surrounding it, thanks to the magic residue left from The Interdimensional War, so if a demon is too weak or too strong it will be unable to withstand being within the human world's atmosphere.
This is the weakest known class of demons. Demons of this class are neither strong nor smart and have only been shown as Noir's grunts, although they are regularly killed by him as well. They are too weak to withstand the Earth's atmosphere, so they are found exclusively within Dispatia. If they were able to go to the Earth, they would still be far stronger than any known human.
This is the average class for a demon. They have average strength and intelligence, although they are leagues ahead of any D class demons. Some of Noir's minion are C class demons, as well, as this is also the class with the most demons. This is the lowest class able to withstand the Earth's atmosphere.
The known C class demons:
This class is where the strength and intelligence really skyrockets. The difference between this class and C class is astronomical when compared to the difference between C and D class. Demons in this class are much stronger than the average person and can easily decimate cities in small groups. This class, while not quite as common as C class, has many different kinds of demons in it, and it is also the strongest class of demon you would find in the human world, as any demon stronger than B class will be unable to withstand being in the human world.
The known B class demons:
The second strongest and most intelligent class, A class demons are few and far between. These demons are almost exclusively found in Dispatia, being that they're too strong to be in the human world. However, with the aid of a medium of some sort, some A class demons have found their way into the human world. The only known ones to have done that are Noir's minions, who were able to reach and stay in the human world thanks to his power.
The known A class demons:
The demons within this class are the absolute pinnacle of power and intelligence. There are very few demons within this class, being that if there were too many S class demons then the balance of power within the world would not be stable. As such, these demons have extremely long lifespans and their lives are integral to the balance of the different worlds. If one S class were to be killed, another power would have to take its place that must be just as strong, else there will be such a cataclysmic effect that not even The Queen knows exactly what would happen.
The known S class demons:
There is also a class all on its own, that of the angels. What exactly the angels are or where they came from is unknown, but there is evidence to suggest that there are many different types of angels. Many believe them to be true angels of the gods, sent down to guide people through life, while many others believe them to be an omen of dark times, sent to bring all of life itself to the brink of death.
There has only been one angel shown: