I wrote it horribly, not according to what I was thinking. I was thinking of the derivative, the rate that stability increases from zero degrees: the slope of the curve. In other words, does stability increase only slowly as the boat begins heeling from zero degrees, or sharply? A single number could give a reasonably good picture here.
Yes, I understand what you mean
You can see that on a GZ (or RM) curve and it is exactly as you say, a step curve indicates a boat that generates RM with small angles of heel. Multihulls have a very step GZ curve, so step that it don't look like a curve
Beamy boats have more steeper curves comparing with narrow boats but, providing they have an adequate RM to sail, they will have also a higher AVS and a smaller inverted stability.