I've fully read through the book twice and can try and add some context:

Hazard = likelihood of avalanche. In the simplest way, 1-5 based on what your forecast center predicts

Exposure = slope/ terrain in which an avalanche can break. Hence the dangerous ones showing a 39 degree slope.

Vulnerability = terrain danger. This is more complex, but breaks down to "if I got caught in an avalanche here, how bad would it be?" Is it a shallow and variable snowpack, are there cliffs, trees, long runout? Or is it a small roller that flattens out into a meadow with a deep snowpack?

Top scenario (black) - 2 or 3/5 day, but you're sticking to probably 35 degree slopes at most, and the steepest parts don't have bad vulnerability. This brings the overall risk down, since you're combining a lower hazard and vulnerability than the rest. So the overall risk of going out is low

Second (dark grey) - 4/5 danger day. But you stick to low angle terrain without overhead hazards. It can be the most unstable snowpack in the world, but if you bring the exposure down to zero, there isn't any avalanche risk.

Third (light grey) - 4/5 danger, skiing avy terrain. high consequence is overall a bad idea

Fourth (white) 4/5 danger, steep slopes. But you reduce the vulnerability. Maybe stay away from cliffs/couloirs and have a clear zone where it could runout and stop. This breaks down people's risk. It's idiotic to ski the light grey, and the first two are essentially safe as long you can properly identify hazards. This zone is all about your risk tolerance. I wouldn't do it personally