Hi lilac2,
The IFD of polyfoam is tested at 25% compression with a 50 sq in round compressor foot on with a 4" thick piece of polyfoam so a layer that is 17 ILD would take 17 lbs of pressure for the compressor head to compress the 4" thick layer down to 3". With a 21 ILD layer it would take 21 lbs to compress the material down to 3".
Compression modulus is the rate that a material gets firmer with deeper compression so if a material has a higher compression modulus it would be firmer if the material is compressed more than 25% of its thickness and softer if it was compressed less than 25% of its thickness. It has a different compression curve in other words. Most people would compress a topper more than 25% of its thickness at least under some parts of their body. This is why higher grades of foam are often used in chair cushions because they can feel soft but they also “firm up” faster so there is less chance of the foam bottoming out.
All the factors that can contribute to how soft a topper feels can be quite complex because thickness and to some degree point elasticity also plays a role (see post #4 here) not to mention the specifics of the mattress underneath the topper (a topper will feel different on different mattresses) so if you can’t test a topper in person in combination with your mattress then I would suggest a phone conversation with the supplier who will be in the best position to help you compare the topper or the type of polyfoam you are considering with other densities and ILD’s of foam.
Phoenix