Self-Proclaimed Demon Lord
Self-proclaimed Demon Lords are simply that, self-proclaimed. They usually have a Rank of A up to Special A in most cases and are either only tolerated by the council of the Ten Great Demon Lords (or later Octagram) due to being too weak to even bother about, or are otherwise a pawn (often without the self-proclaimed Demon Lord even noticing) or target of interest to one or more members of the council. If none of these conditions apply, then it's likely that the self-proclaimed Demon Lord will soon be subjugated by the council for daring to call itself a Demon Lord.
Sub-Demon Lord Class and Demon Lord Class
Sub-Demon Lord Class refers to individuals on the lower end of Special A-Rank. Individuals like Phobio and the other Beastketeers are considered as such. They are often either one of the stronger self-proclaimed Demon Lords or otherwise the second-in-command of a Demon Lord Class individual or Demon Lord Breed.
Demon Lord Class refers to individuals who have fulfilled the strength and Magicule requirement of a Demon Lord Breed, but might not have necessarily fulfilled the other conditions yet. They are usually ranked on the higher end of Special A, possibly even reaching the lower end of S Rank.
Demon Lord Breed
Demon Lord Breed (魔王種 maō shu, lit. "Demonic King Breed"), sometimes also referred to as Demon Lord Seed are those who have fulfilled the necessary conditions to be able to start the Harvest Festival. In terms of strength and Magicules, they are Demon Lord Class, i.e. between high Special A-Rank up to low S Rank.
Depending on the individual, becoming a Demon Lord Breed by fulfilling all the conditions to become one, might cause an announcement by the Voice of the World and a race-change, while for others neither evolution nor announcement happens, though they are still Demon Lord Breeds regardless.
The reason some Monsters go through this evolution on Demon Lord Breed stage is likely due to their current species not being able to handle the power of a Demon Lord Class, as could be seen with the Orc Lord being confused and severely limited with his actions due to being unable to bear the power obtained through Starving Ones until he became an Orc Disaster who could handle the burden. Rimuru likely didn't go through any evolution because Slimes are innately a race close to Spiritual Beings and have good Magicule conductivity, on top of which he got his Stomach sub-space that possesses immense capacities. So an evolution wasn't necessary and would have only been a waste of Magicules. It's unknown if those who go through such an early evolution also go through another race-changing evolution upon becoming True Demon Lords, or if they just gain the benefit of additional Skills and enhanced Magicule capacities.
Since Demon Lord Breeds are required to harvest human souls for the Harvest Festival, they require the ability to interact with and collect Souls of those they kill. It is unknown whether one of the conditions of becoming a Demon Lord Breed is being able to gather and/or interact with souls, or if becoming a Demon Lord Breed is what grants that ability. Regardless, it can be assumed that Demon Lord Breeds are definitely capable of gathering the Souls of those they kill.
The Harvest Festival (
During the Harvest Festival, the one ascending undergoes a transformation in which they can physically and racially evolve, and gain a tremendous amount of Magicules, notably ten times as much as their previous amount. Additional abilities and Ultimate Skill can be obtained during the transformation. There is also the possibility of a significant change in personality.
Additionally, the subordinates of the one who ascends will also receive "gifts" of power or ability.
True Demon Lord
True Demon Lord (真なる魔王 shin naru maō, lit. "True Demonic King") is one of the ultimate forms of existence. It is both a title and a race, since True Demon Lords usually become full Spiritual Beings by fusing their material body and their spiritual body, making them able to switch between them as they like without having to worry about the rejection force that the Spiritual Beings from the Spiritual Plane usually experience without a material body or other kinds of anchor.
According to the Voice of the World, the title Demon Lord always refers to True Demon Lords, meaning that such an existence is approved and acknowledged by the World's Systems itself, while Demon Lord Breeds are clearly referred to by the Voice of the World as "Demon Lord Breed" instead. The only reason the term "True Demon Lord" even exists is for individuals to distinguish them from the other definitions of the term "Demon Lord."
It is extremely difficult to fulfill all the conditions to reach this ultimate form of Monster-hood. Even those who have lived for many eons are not guaranteed such status, either because they couldn't properly increase their strength or Magicule capacity, or because they failed to fulfill the conditions for Demon Lord Breed and/or True Demon Lord. Orc Disaster was in the process of becoming one but was stopped in the middle of his transformation since he was defeated before he could fulfill the final condition of collecting 10,000 Human Souls.
True Demon Lords usually Range from higher S Rank to Special S Rank.
Known True Demon Lords
Those who are known to be True Demon Lords by the time of Light Novel Volume 8:
Recognized Member of the Demon Lord council
Founded by Guy Crimson, the Demon Lord council is an organization where all the notable Demon Lords of any kind gather. The number of members and the name of the council varied greatly throughout history, but at one point the maximum number of members was capped at ten and the name "Ten Great Demon Lords" became established. However later the organization was restructured again during a Walpurgis Banquet, and so the resulting organization was labeled as Octagram by the "newbie" Rimuru Tempest.
The only condition required to join the council is to be accepted by its members or forcefully take a spot from one of them. Anyone who joins is automatically considered at least S Rank just for being a recognized member of the Demon Lord council, regardless of actual power unless it's Special S. However it's pretty much impossible for anyone other than Ramiris to join the council without being at least at Demon Lord Class of power. Furthermore, the Octagram got even higher standards since every member is already at least True Demon Lord level of power.