What is the advantage of using negative enums

Question

In the example of the kaleidoscope / AST analyzer in LLVM, the enumeration receives all negative values. Why is the minus sign?

enum Token {
  tok_eof = -1,
  // commands
  tok_def = -2, tok_extern = -3,
  // primary
  tok_identifier = -4, tok_number = -5
};

Answer 1

C- . , , , , - "undefined". , tok >= 0.

Answer 2

, .

0 255, tok_eof. , 0 255 , , 256, 257, 258 .. , 256, 257, 258 IMO.