People call this place "the world";
In the "world",
People are divided into S. A. B. C. D levels according to their strength, and are further divided into 0.1.2.3.4.5.6.7.8.9 And so on.
The highest is S/0 and the lowest is D/9.
People here are born knowing the purpose of existence:
Survive, kill, higher levels