This was what was written in slack.

The reason why there aren't absolute Y values is because that would make dealing with edge cases difficult. Instead, we can use Basement Floors:

G: Ground Level

L1: Tunnels that connect to both roads above and marketplace below. Can comprise of subways as well

L2: Market Zoning at the center of city. Industrial & Residential outside the central, maybe.

L3: Railway and maybe ice roads

L4: Pipelines

L5: Misc.

Bedrock: Vaults and banks (edited)

I would recommend against strictness. If someone asks for a massive pit to farm something, or to make something like the hexagon, sufficient land can be cordoned off. But I guess in the absence of prior plans, we can defer to this vertical layout

While boroughs are free to do whatever they want, hopefully they would agree to the generics of this zoning plan. We should leave it up to their own internal debate. If basement levels are different Y levels between districts, that's fine, since what is ideal is that the Y levels connect to one another in order to ensure that the underground is contiguous.