The group known as Demon Lords 「魔王, Maō, lit. "Demon King"」 are those beings that have great strength (Disaster Rank A and above) and have a great deal of notoriety and/or also are part of official Demon Lord organizations such as the Ten Great Demon Lords.
Self-proclaimed Demon Lords are simply that, self-proclaimed. They have an EP of an A Rank monster and have close affiliations if not part of the organization of Demon Lords. Karion, Frey and Clayman are such examples. Some individuals can even be called a Demon Lord even if they're not part of the officially recognized organization. A brief example would be Rimuru, who, while massacring the Farmas Kingdom army, was thought to be a Demon Lord even if he wasn't one (yet).
Above this, legendary existences are the True Demon Lords who are of Disaster Rank S and above. They are those who have had the Demon Lord's Seed in them sprouted and thus became recognized as a Demon Lord in its truest meaning by the Voice of the World.
Demon Lord Evolutionary Process
An Individual who can claim to be a Demon Lord is one whose Magic Amount (EP) has reached or risen beyond 200,000. Most, if not all, of the self proclaimed Demon Lords are such Individuals. However, even then if the Individual themselves dares to actually declare themselves a Demon Lord without being acknowledged by any existing ones then they'll quickly find themselves being crushed by the already established lords. As such anyone who'd lay claim to the title must first gain the support of at least two existing Lords.
Different from these self proclaimed types are the True Demon Lords, these are beings who have sprouted the Demon Lord Seeds within themselves by collecting a vast amount of human souls as nourishment. Upon reaching this level one undergoes a Harvest Festival, which strengthens any and all of their followers, and a personal transformation. After this evolution their Magic Amount will raise exponentially to a state roughly ten times greater than before, meaning more than 1.000.000 at minimum, and they will obtain an Ultimate Skill. A good example for this is Rimuru Tempest himself who according to Milim was already a worthy Demon Lord when they first met days after his fight with the Orc Disaster Geld. This would mean that Rimuru had more than 200.000EP even then an likely more than 2.000.000EP following his harvest festival.
Furthermore the harvest festival itself can promote a True Demon Lord's subordinates to a level great enough that they can rival and surpass the self proclaimed, as shown by Shion, who at the time of the Walpurgis Banquet arranged by Clayman had a Magic Amount of 224.000EP, which was decidedly higher than even Clayman's himself.
Self -proclaimed Demon Lords have risen and fallen all over history and as such the Eastern Empire thinks that a demon lord is not truly a big deal. Indeed even a powerful Knights Templar can defeat such Individuals with the use of Legend Grade Equipment. However the True Demon Lords are a completely different animal, rarely seen and legitimately powerful.
Finally, should a True Demon Lord's subordinates also become True Demon Lords, he can become a Great Demon Lord, or at least this is the case for Rimuru who was able to power up from his cabinate class subordinates, who were in the process of this evolution, via his Gluttonous King Beelzebub's Food Chain ability. This ultimately pegs Rimuru's EP at this time around at least 24.000.000, and more than likely much higher than that due to each of those newly risen True Demon Lord's granting Harvest Festivals unto their own retainers.