A sub level is what you get when you reach level 1,000. In total you can get 10,000 sub levels, 9,000 more than what you could originally get. Every sub level requires five million (5,000,000) experience, which, to be honest isn`t that much. Every time you get a sub level you get a point, which can be used in one of five things: Strength, Dexterity, Speed, Intelligence and MaxStamina. There`s also two titles you can get which involve the sub levels, one of them is Legend. This title was added when the sub levels capped at 1,000. So if you want the title, get 1,000 sub levels. The second title is Demi God, which is unlocked when you get ALL 10,000 sub levels.
Now you must wonder, being a high level player, there isn`t many places to get sub levels right? Wrong. At first, when that capped sub level was 1,000 players used Legend Dungeons to level up, aswell as the Battlefield. Nowadays though, you get more experience for gathering and crafting level 1000 materials, and the new realm, Eternal Plains, has higher level monsters to test your skill, but give quite the worth while experience. Ofcourse, this realm isn`t unlocked until you`re level 1,000.
So get out there and work hard for your sub levels!