Hi thecountofzero,
Do you mean the 7 zones in the Spindle layers?
In theory can help provide different levels of support under different areas of the body. For example a softer shoulder zone can “allow” the wider lighter shoulders to sink in more deeply while a firmer zone under the pelvis can help prevent it from sinking in too deeply. The success of a zoning scheme depends on how well it fits the body type of the person but a suitable zoning scheme can help with more difficult body types or body shapes.
There is more about zoning in this article and in post #11 here and in post #18 here.
I think dn’s suggestions about asking Spindle may be the best solution to find a similar material to what they use in the WJ Southard layer covers but there are also some sources for topper covers in post #4 here although these would probably be thicker and meant more for toppers and not so much for covering layers inside a mattress.
Phoenix