This is the 65% IFD (how much weight it takes to compress a material 65% of it's thickness) divided by the 25% IFD. It is a measurement of the progressive resistance of a material. Higher quality materials will have a higher progressive resistance and although it is less well known or talked about in the industry, it is at least as important as ILD in determining the comfort and support qualities of a material.

AKA: Comfort Factor