Hi amun,
They are firmness ratings that Mountaintop foam uses for their latex. Each firmness rating (from C0 which is the softest to C5 which is the firmness) has an ILD range that is associated with it although their ILD ranges don’t match the ILD ranges of other latex manufacturers that make different types and blends of latex. There is more about ILD ratings in post #6 here. It’s certainly not something that is particularly important to know
Phoenix