Hi idlewild,
Zoning can be part of any layer or component in a mattress but it’s more common in support layers.
When someone has more challenging circumstances or finds it more difficult to find a mattress that is a good “match” for their body type and sleeping positions or because they have a narrower comfort or support range that seems to work well for them then zoning may be helpful because the firmer zones under the heavier parts of your body can allow you to use softer comfort layers with better pressure relief with less risk to alignment. There is more about the pros and cons of zoning in post #11 here and the additional links it includes.
Phoenix