It’s related to a ratio. A ~6 degree slope would have you dropping one meter for every ten meters you move forward: 10%. 45 degrees means one meter for every meter forward: 100%. Keep plotting that out and it approaches infinity as you reach 90 degrees (or just ask, “for every meter forward, how many up/down?” if it is 90 degrees.)

The problem is that "forward" is ambiguous. You can measure it relative to your direction of travel, or perpendicular to the gravitational vector. For small gradients these are more or less the same, but the steeper the slope the more these two measures diverge.

Fortunately, as 110% it's not ambiguous anymore.

With slope in degrees and grade in percent,

100*tan(slope) = grade%

slope = arctan(grade%/100)

so 110% is about 47.7 degrees, and tan(90 degrees) is infinitely large.

Precisely. Math makes it clear to understand and settle down the discussion, thanks ;)

It would be, yes, basically.

If you have a 90° angle that would be called a “wall” not a “slope”.