Rank

Background
Both monsters and adventurers are ranked from F to A. Monsters who are in the upper echelon of their rank have a "+" attached while those in the lower echelon have a "-" attached. World Traveler Grand Master Yuuki Kagurazaka established this ranking system in place of the previous 4 step Novice -> Beginner -> Intermediate -> Advanced system.

After the introduction of the letter based ranking system, Novice was replaced with F, Beginner with E, and Intermediate with D. Meanwhile, those who were previously categorized as Advanced were split further into C, B, and A. Doing so distinguished the stronger from the strong. 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; a monster that deviates from the standard ranking system, an abnormal existence. At present, an A class monster is capable of jeopardizing the existence of a country, a strength capable of inducing despair.

The new ranking system is built on the inference that a party of adventurers is taking down a single monster of the same rank. For example, if adventures are going to hunt a rank B+ monster, multiple rank B+ adventurers are needed (At least 3 people). 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 doesn't mean that the subjugation can begin immediately. Normally, not many rank B+ adventurers stay at the frontier; the problem is that monsters 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 knight order. If you are planning on subjugating such a village with rank B- knights, you'd need about 3,000 of them.

A Rank Humans is considered a Heroes by Human Countries.

B Rank and below adventurers aren't famous enough to be known outside their country's borders.

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

The amount of EP is only the required amount for obtaining a "Rank" for the species. Individuals like Veldora or Guy Crimson 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 the rank for Monsters are decided by their Magic Amount or EP. Thus, monsters of the same rank can have massive differences in 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, exceptions being those who have gone through Evolution.

Disaster Rankings (Web Novel)

 * *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:
 * 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.

Rimuru noted that if there are only three stages, the gap between each stage should be large, to which it 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.

Also in the 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 Web Novel. Therefore, somethings might not make sense but this is done in order to keep the confusion to minimal as the Web Novel 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 Web Novel 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.