Hi donw,
The zoning design that works best for individual people can vary widely and can be different for different body types and weight distributions. There is more about zoning in this article and in post #2 here and post #2 here and post #4 here.
With a simpler 3 zone mattress then the middle zone would generally cover both the hip/pelvis and lower back areas and would generally need to be firmer to better support this part of the body which carries the most weight.
With 7 zone configurations that have more zones that can target more specific areas of the body the zoning system that would tend to work best “in theory” for the largest number of people (keeping in mind that theory doesn’t always reflect the ideal combination for any specific person) would generally be softest under the shoulders, a little firmer under the hips/pelvis (the hips are a pressure point for many people but are also heavier than the shoulders), and then firmest under the recessed part of the lower back/lumbar area.
Having said all that … some good research and studies (by Bart Haex in particular) shows that the “ideal” zoning configuration is very individual (and some body types may do best with no zoning at all) which is why “theory” may not be relevant for a specific person and personal testing or your actual sleeping experience is the only reliable way to know whether a particular zoning configuration will work well for any specific person.
Phoenix