How to divide the hardness of foam?

When we see or touch foam, we will find that different foams feel different. Some will be harder and some will be softer. Why? Because foams are generally expressed in terms of density, the higher the density, the greater the hardness. However, due to the different uses of foams, the hardness of sponges with the same density will also change after changing the batching method (for example, 35 density can be divided into 35 soft, 35 medium soft, 35 hard and 35 plus hard). Under the same density, the softer the elasticity is, the better the hardness is, and the worse the elasticity is.