Hi WadeB,
C3 is a firmness rating. Mountaintop uses a firmness scale that ranges from C0 to C5.
You can see the ILD range for each of their ratings here …
The C5 isn’t on this list and it’s 33.5 - 37.5 ILD.
I would be cautious though if you are comparing their ILD ratings to other types or blends of latex that you are familiar with because they may not match very well and there are also other factors besides just ILD that will determine how soft or firm a material “feels” when you lie on it (see post #4 here). The best way to assess how soft or firm a material (or a mattress) feels for you would be based on personal experience.
They have 7 zones and if you look at sdmark’s last picture here you will see sections of the latex that have smaller pincores and sections that have larger pincores. The larger holes are the softer sections and the smaller holes are the firmer sections. The shoulder zone (second zone from either end) is the softest and the middle zone under the pelvis would be the firmest.
Phoenix