Rank

Background
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 a 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.

And by the way, Advanced is C rank and above, Intermediate is D rank, Beginner is E rank, and Novice is 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 be 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.

Difference Rank between Human and Monster
The Rank for Humans and Adventurers are decide by their Skill while Monsters are decide 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.

Manga
In the manga, however, the ranking system for monsters and the like seems to be different. Till now they've only revealed four levels. In ascending order, they are: And a special rank called: Rimuru himself asked that since there are only three stages, the gap between each stage should be large, which is when it was revealed to him that there was the rare Catastrophe rank people. But because there are so few people in that rank, everybody pretends it doesn't exist.
 * Hazard
 * Calamity
 * Disaster
 * Catastrophe

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