there is no such thing as a level 0 building. it's either level -1 or level 1.
Alright then. Please let me rephrase my original question to put aside any semantic issues and restate the original inquiry.
Is the structure a subsurface or semi-subsurface (or even just a regular structure) structure given the following two scenarios:
A: The structure's highest level is level 0 (with the roof visible to aboveground units, but not extending above the surrounding terrain), but may have ammo storage, quarters, or other facilities extending to deeper subterranean levels that cannot be seen or interacted with by aboveground units.
B: The structure's highest level is level 0 (ditto above), but does not contain any levels or hexes that cannot be seen or interacted with by aboveground units.