Rereading I think I misunderstood the question and too tired to confirm now.
I'll check again in the morning if it's not answered
So my understanding is that you own the parcel your land is built on, and the surface it contains.
But (and I could be wrong here) if your surface moves through two z-axis parcels you would then own them both whilst only owning the one parcel on the x and y axis.
In other words you bought the surface and so for your parcel it doesn't matter how many z axis it goes through.
For the air and subterrain parcels this is a mute point. You would own the whole square