Italic Angle in the Font Info Key Dimmensions: Positive or Negative value?

I'm making some macros that make use of the italic angle font info data.

While testing the macro, I've found that some fonts use a negative value.
For example, if the angle is 12°, some fonts use the value: 12 (positive) while some other use the value: -12 (negative)

Is there any reason to use a negative value?
Is it documented somewhere?

Thanks in advance