Hi gu-obos,
All mattresses or sleeping systems include a top section (or top few inches) that acts as a comfort layer or “zone”, a bottom section or “zone” that acts as a support layer or “zone” and a middle section or “zone” that acts as the transition between the two. Even a mattress that only has a single layer has a “comfort layer” although in this case it’s not a separate layer but the top few inches of the mattress (see post #2 here). The terminology of “comfort layer” is really for ease of reference in other words and the comfort zone (the depth of the pressure relieving cradle) may not be the same as any of the actual layers. The difference between different “comfort zones” is in the type of materials, the softness, and the thickness of different types or combinations of “comfort layers” regardless of whether they are inside or outside the mattress or are even a pillowtop which is basically a topper that is attached to the mattress (and can’t be removed).
The two main functions of a mattress or sleeping system are support/alignment and comfort/pressure relief and everything after this is really a preference. It really makes no difference if a sleeping system includes a functional box spring, a mattress, or a topper, or any combination of the three as long as the combination used in the sleeping system is suitable for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Using different combinations of components and materials is how a mattress is “matched” to different body types, sleeping styles, and preferences and the end result is always more important than the combination that creates it.
A mattress / topper combination can also have advantages over the exact same layers inside the mattress cover (where the same material that is used in the topper is inside the cover instead of being a separate layer on top) because it can be changed or replaced without having to replace the entire mattress if your needs and preferences change over time or if the top layer of your sleeping system softens or breaks down sooner than the rest of the mattress (and a mattress will usually soften and break down from the top down). A topper will also extend the life of the layers below it because it will absorb most of the compression forces when you sleep on the mattress.
The challenge of course is choosing a topper that in combination with your specific mattress is the most suitable combination in terms of PPP because choosing a topper that works with a particular mattress can be almost as difficult as choosing a mattress if you cant test the specific combination in person. Many manufacturers even design their sleeping systems as a mattress/topper combination because of some of the advantages of using a more “modular” design.
Phoenix