Rank

Rank (ランク) refers to the strength and danger evaluation system of the Free Guild for Monsters, Adventurers, and other groups.

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. Grandmaster Yuuki Kagurazaka established this ranking system in place of the previous four 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.

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 adventurers 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 the rank B+ monster, 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.

Rank A- and above humans are considered "Champions" by Human countries.

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

In the case of most monsters, the amount of magicules is the only measurement scale for obtaining a Rank. However, monsters with lower magicule storage like Hakurou, who should only be at the level of regular A-Rank, manages to reach Special A Rank for the standards of human adventurers by proficiency with the sword alone. For the higher ranks of monsters, a monster's level of threat is more important than its raw power. That's why Charybdis, despite having Disaster-class (Rank S) magicule capacity, is only considered a "Special A-Rank," since its lack of intelligence makes it nothing more than a mad beast.

Differences about Ranks between Human and Monster
The Rank for Adventurers are decided by their proficiency, while the Rank for Monsters are decided by their Magicules. Thus, monsters of the same Rank can have massive differences in power.

It should be noted, however, that this measurement system is flawed and there are many ways for someone to be actually stronger than their Rank suggests; for example, intelligent monsters can learn human "Arts" or possess Skills that grant them more effective battle power than a greater Magicule Capacity would grant. On the other hand, humans can become Sages or Majins and make contracts with Spirits. Otherworlder humans also have their own advantages, such as Skills (possibly even Unique Skills) or Magicule levels that regular humans can't achieve normally.

Threat Levels
For Rank A and above, monster ranks are evaluated based on their ability to threaten civilizations rather than just their individual magicule amount. There are 4 threat levels.
 * A Rank (A級), also known as Hazard-class (災害級)
 * A threat that could potentially cause widespread damage to a single town or region.
 * Includes the Ranks A+, A and A-.
 * Special A Rank (特A級), also known as Calamity-class (災厄級)
 * A threat that could topple a country's government. This level includes high-level Majin and demons, as well as the infamous Charybdis.
 * S Rank (S級), also known as Disaster-class (災禍級)
 * 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.
 * Demon Peers are unofficially recognized as this at the minimum.
 * Just being a Demon Lord Seed by itself is not enough to qualify for Disaster-class, as for example Orc Disaster Geld, only advanced from Hazard-class as Orc Lord to Calamity-class as Orc Disaster. However, being accepted into the Demon Lord Council immediately warrants the newcomer to be considered S Rank.
 * Special S Rank (特S級), also known as Catastrophe-class (天災級)
 * This level applies only to some extraordinarily powerful demon lords, as well as True Dragons, 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.

Ranks of Notable Individuals
Rimuru:
 * Hazard-class: Despite Rimuru's early evaluation of himself as someone who is supposedly Rank B, he was actually already at Rank A when he was reincarnated as a Slime, even before being named.
 * Calamity-class: After being named by Veldora, and predating several monsters, Rimuru already reached a magicule level that should at least be comparable to Ifrit, who was a Calamity-class himself at that time. After predating Orc Disaster Geld, Rimuru already was above the so-called "Sub-Demon Lords" like Phobio, but not Disaster-class yet.
 * Disaster-class: Either after predating Charybdis or the Sky Dragon, Rimuru reached Disaster-class. That is proven when Razen, a credible source as far as threat levels go, evaluates Rimuru as such. Rimuru was officially recognized as a Disaster-class threat by the public once the Demon Lords' reshuffling from Ten Great Demon Lords to Octagram was announced.

Trivia

 * During the first introduction of the upper threat levels, Rimuru was only told of Hazard-class, Calamity-class and Disaster-class. So Rimuru noted that if there are only three stages, the gap between each stage should be quite 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, most don't bother even bringing it up.
 * The power level of individuals within the S Rank/Disaster Class varies by a lot. The lower end consists of Demon Lord Seeds like Frey, Carrion and Clayman, whereas the top end holds powerful True Demon Lords like Luminous Valentine or Ex-Heroes like Leon Cromwell. The difference can be more than 10 times as much in magicule capacity alone.
 * The description of the threat levels are written in the Western Nation's perspective. The Eastern Empire could've easily handled multiple Disaster level threats at the same time.