Roof:angle definition

Could You please help me to understand how the roof:angle is defined? Maybe the answer is very obvious, but I haven’t found the clear explanation neither in wiki nor while studying real usage cases on the map, hence I can’t figure out which angle is taken into account while rendering. Let’s consider two most simple roof shapes: gable and skillion. Is the roof:angle:
a) the one between a horizontal line and the sloped roof surface?
b) the one between a vertical line (e.g. building wall) and the roof?
c) or maybe for the symmetrical gable roof this is the “tip angle” between two sloped surfaces? (which would mean two times answer “b”).
d) maybe some other possibilities?
The only clues I’ve come acrross is here and it suggests that answer “a” is correct but does it actually refer to all roof shapes?

I think its (a) … so roof:shape=flat has a roof:angle=0 … but I don’t know any rendering engine who make use of it.