The common factor in all differential constructions is that they use a comfort layer that is THICKER than the recessed areas or "gaps" in a sleeping profile. If for example you need a pressure relieving cradle that is 3" deep (average side sleeper), then a differential construction could have a comfort layer that was at least 3" thick and usually a little more. In this type of construction, the comfort layer is designed to do all the work of forming a pressure relieving cradle and supporting the recessed lumbar area and the support layers below it are only used to keep the spine in alignment by preventing any further sinking down by the heavier parts of the body. This means that in this construction, a comfort layer needs to have all the qualities that are necessary by itself as it does not “borrow” qualities from the layer below it. This means that for pressure relief, it needs enough softness and point elasticity, and for lumbar support it needs a higher sag factor and resiliency. In other words, if you needed a 3” cradle for good pressure relief then you would use a comfort layer that was about 3” - 3.5” thick. If you needed a 2” cradle for pressure relief, then you would likely choose a comfort layer that was 2” – 2.5” thick. The comfort layer would always be at least as thick as the cradle that you need and usually a little thicker.
Differential constructions are much more simple than progressive since they use the qualities of a single layer rather than combinations. Since there are so many high quality materials available that are good in all the areas required for a comfort layer, it is simply a matter of choosing an appropriate thickness and ILD and thickness of a high quality material. The other qualities that are needed (point elasticity, sag factor, and resilience are already a side effect of this choice. In this case, latex, HR polyfoam, and microcoils which all have versions that have lower ILD, high sag factor, high resilience, and good point elasticity that are necessary in a good comfort layer. In some cases collapsible column gel may also make a good choice here since it also has the “built in” ability to support the lumbar (with the stiffer columns that have not collapsed) but may not be available in a thick enough layer in which case it would need a progressive construction. Because the comfort layer is based on a single material over a firm support layer, it is much easier to “get it right” than the more complex progressive approach.
They can use a much wider variety of support layers, including lower cost materials, because the main quality of a support layer that is needed in this type of construction is firmness to control the sinking down of the heavier parts of the body and any support layer can be made in a firm version.
In a 3 layer mattress, it becomes much easier to choose the middle layer (part of the support core) as it only needs to be similar in firmness to the bottom layer so the overall support core is “firm”.
This type of construction depends on the use of comfort layers that are higher quality and have many qualities so the choices of material in the comfort layer is more restricted. It is also important to remember that there are some materials that can feel very good and have many of these qualities such as softness, point elasticity, and resilience at a lower level and for a short time (such as lower grade polyfoam) . While these may feel good in the store and for a short time afterwards, they will quickly lose their qualities and good dreams may turn into nightmares. In a differential comfort layer that feels good when you are testing it, knowing what it is made of and it’s expected durability becomes very important.
This type of construction should not be used at all with certain comfort layers such as natural fibers and should be used with real care if at all with others that have weaknesses in certain areas such as memory foam where they are usually only used in certain cases (such as those few with thin very evenly distributed body profiles that do not require the same degree of targeted lumbar support and do not have a tendency to sink down too far in certain areas of their body but still need good pressure relief overall).
Overall, this would be the preferred construction method for those who wish to simplify their mattress shopping or those who are buying a mattress without lying on it first. It is much easier to predict what a mattress of this type will feel like based on what other mattresses with similar constructions and layer thickness feel like in the stores. In other words, a mattress with this type of construction can be more easily duplicated with other materials and through other suppliers which opens up great possibilities for greater value in your mattress purchase.