Class Level is divided into seven Ranks, namely Low Rank, Mid Rank, High Rank, Grandmaster Rank, Legendary Rank, Demigod Rank, God Rank.
The Level ranges for Low, Mid, High, and Grandmaster Ranks are Level 1 to Level 5, Level 6 to Level 10, Level 11 to Level 15, and Level 16 to Level 20 respectively. As for Legendary Rank, it is Level 21 and above.
Demigod Rank is a unique, separate group. This is because they are immortal and possess frightening strength. Hence, they must be placed in their own unique system.
Demigod Rank has a total of 10 levels. A Level 1 Demigod Rank character is equivalent to a Level 26 Legendary Rank character and a Level 10 Demigod Rank character is equivalent to being Level 35 in Legendary Rank.
God Rank is above Demigod Rank. In other words, it's equivalent to Level 36 and above in terms of Legendary Rank.
Some information relating to God Rank.
Blizzard's setting is simply too disorganized. Demigods, Loa, Old Gods, Gods, and the Titan Pantheon are all not true Gods. There are also the Naaru and other messy things. So, it's impossible to follow Blizzard's lore faithfully.
Hence, I've set the strongest boss to be God Rank, such as the Void Lords and the Titans.
As for the Old Gods, they are nothing but servants of the Void Lords. They were dispatched to corrupt Titans that have yet come to existence, but they failed to do so after a long time. The strongest Old God, Y'Shaarj, was single-handedly crushed to death by one of the Titans. Hence, the Old Gods are not God Rank. They're considered Demigod Rank.
Many friends of mine are gamers and they requested that Class Level match with Game Level. Generally speaking, this is the conversion rate. Class Level 1 is equivalent to 5 levels in the game.
In other words, a Level 1 Mage in the story is equivalent to a Mage between Level 1 to Level 5 in the game. A Level 5 Mage is equivalent to a Mage between Level 26 to Level 30 in the game. A Level 20 Archmage is equivalent to Level 96 to Level 100 in the game.
However, I wish for everybody not to compare it down to the dot. This is because the game has considered its nature and there are many things that simply don't make sense. For example, VanCleef's Deadmines is a Level 10 Dungeon, and yet his daughter, Vanessa's Deadmines is a Level 85 Dungeon… How does that make any sense?
Hence, when it comes to the Dungeons from the game itself, I'll merge my ideas with Blizzard's setting and adjust the Level accordingly.
For example, Arugal from Shadowfang Keep is an Archmage and he has Ancient Magical Worgens as his subordinates. This means that Shadowfang Keep isn't something a group of Level 10 to Level 20 can defeat.
As for the Combat Level.
Combat Level is an individual's actual combat strength. It's a calibration towards the individual's Class Level.
For example, a specialized Holy Priest, even if his Class Level is equivalent to an Archmage, might not be able to defeat an experienced old soldier.
On the other hand, an experienced Assassin can kill a Mage that is much higher level than him.
For instance, the main character might only be a level 2 mage, but he has a fully-equipped gear. Hence, his Combat Level would always outrank his Class Level.