Rank

Fondo
Both monsters and adventurers are ranked A~F–all in those 6 categories. Those especially strong have a "+" attached, those weak have a "-". This was guidance from the World Traveler, Grand Master Yuuki Kagurazaka. He established this ranking system. Up until then, it was Novice –> Beginner –> Intermediate –>Advanced; a four-step system that was easy to understand.

Also, Advanced is C Rank and above, Intermediate is D Rank, Beginner is E Rank, and Novice is an F Rank. Thus, due to the introduction of the new system, the number of deaths due to fighting stronger than expected monsters greatly decreased.

"+" and "-" only appear at A, B and C Rank.

Those of the same rank would be a match in a one on one. Thus, fighting in a party became safer. Surpassing A rank is the specially designated (True) Demon Lord level S rank, and above even this stands the monster with the strength of a Natural Disaster, Special S rank. Deviating from the standard ranking system, an abnormal existence. At the present, an A class monster is capable of jeopardizing the existence of a country. A strength capable of inducing despair.

If adventures are going to hunt for rank B+ monsters, multiple adventurers of the same rank are needed (At least 3 or more people). Even if 10 adventurers with rank B tried to subdue it, even if they were successful, there would surely be casualties. Furthermore, just because a sighting was confirmed, it didn’t mean that the subjugation can begin immediately. Normally, there are not many rank B+ adventurers stay at the frontier; the problem is that monsters would appear too frequently.

The ogre village, though perhaps it was too small to call it that, only housed 300 ogres (300 B rank monsters). That is the size of a country’s knights order. If you are planning on subjugating such a village with rank B- knights, you’d need about 3,000 of them.

B Rank and below adventures aren’t famous enough to be known outside their borders.

A Rank Human is considered as Hero by Human Countries.

Although Humans can only be ranked up to A Rank Other exceptions are Chloe O’Bell, Leon Cromwell, Rudra Nam Ul Nasca since they became True Heroes that can compare to True Demon Lords.

The needed amount of EP is only called for obtaining a "Rank" for the species, individuals like Veldora, or Guy Crimson, etc; can exceed their respective ranks via training and other methods.

Difference Rank between Human and Monster
The Rank for Humans and Adventurers are decided by their Skill while Monsters are decided by their Magic Amount, EP & not by their skill thus even though there are Monsters with the same A Rank, sometimes there's a very large difference in their power.

While the highest Rank for Monsters is Special S, the Highest Rank for Humans is only A Rank, there aren't any A+ Rank Humans no matter how strong s/he is.

Disaster Rankings

 * *It must be noted that characters were assigned Ranks and EP values (e.g. Chapter 74) instantially and that a clear chart of EP to Rank categories was never provided. Meaning, readers are prone to guessing in such cases.

Light Novel & Manga
In the LN & Manga however, the ranking system for monsters are only applied for determining their power level for destroying a nation. Till now they've only revealed four levels. In ascending order they are: And a special class called: Rimuru asked that if there are only three stages, the gap between each stage should be large, which then was revealed to him that there exists a rare "Catastrophe" class. But because there are so few people in that rank, everybody pretends that it doesn't exist.
 * Hazard-equivalent ranks to A (i.e. A+,A  & A-)
 * A threat that could potentially cause widespread damage to a single town or region.
 * Calamity-equivalent ranks to Special A
 * A threat that could topple a nation’s government, caused by the maneuvering of high-level magic-born and demons.
 * Disaster-equivalent ranks to S
 * Normally applied to demon lords. Small nations would have no chance against such a threat, and a larger one would need to expend all its resources to handle it.
 * Catastrophe, which is equivalent to Rank Special S
 * This could be applied to some demon lords, as well as dragons and their kin, and reflected the kind of threat that no single nation could handle. It would require international cooperation to give the human race even a chance at survival.

Also. In Manga, Tempest is considered a Disaster rank bunch, and Milim Nava was revealed to be a Catastrophe rank.

Trivia

 * EP means Energy Point(s).
 * Note: All of this is based on the WN. Therefore, somethings might not make sense but this is done in order to keep the confusion to minimal as the WN itself comprises many contradictory statements.
 * Also, the "+" and "-" on A, B & C are only to distinguish different powerful individuals residing in the same Rank. Therefore, it can be concluded that they all fall in the same Rank as a Whole,i.e. Rank [A, B & C] "+" or "-" all fall in Rank [A, B & C] respectively as a Whole.
 * In WN when an individual evolves into a True Demon Lord, his/her EP becomes about 10x (times) of his/her EP preceding the evolution. However, the individual may or may not obtain an Ultimate Skill.

Reference
pt-br:Rank Ранг