Hi lilac2,
It’s actually not quite correct. All the layers of a sleeping system will all compress simultaneously but they will compress to different percentages of their thickness depending on their relative softness, thickness, and their position in the mattress. The layers closer to the top will generally compress more than the deeper layers and softer layers will compress more than firmer layers. What you want is for the combination of layers (mostly in the upper layers) to compress “enough” so that the pressure relieving cradle on the top of your mattress is deep enough to redistribute your weight and relieve your pressure points. Too little and you may still feel too much pressure and too much and you may have alignment issues.
I would keep in mind that a topper is for pressure relief … not support (which comes from the deeper layers). It’s job is to “allow” the heavier parts of your body where you are experiencing pressure (generally hips and shoulders) to sink in more deeply so that the pressure can be relieved.
How deeply you sink in and how effectively your topper will relieve your pressure points will depend on the topper, on the specifics of the mattress underneath it, and on your weight, your weight distribution, your body shape/surface area, and your sleeping positions.
There are too many variables to be able to predict with any certainty whether any mattress/topper combination will be a suitable match for someone in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) without trying it in person but post #2 here and the topper guidelines it links to can help you use your experience on your mattress to choose the thickness and firmness of a topper that has the best chance of success.
I would also keep in mind that ILD alone isn’t the only factor that determines the softness of a layer (see post #4 here) and that ILD isn’t comparable between different types of materials such as latex and polyfoam (see post #6 here) or between different grades of polyfoam that have a different compression modulus so I would always talk to a supplier about the firmness/softness of any polyfoam topper you are considering because going by ILD alone can be misleading.
The goal is that a topper is “just barely enough” in terms of thickness and firmness that it relieves your pressure points in your most pressure prone position (generally side sleeping) but isn’t so thick and soft that it causes you alignment issues in any other positions you sleep in (back and stomach sleeping).
Phoenix