Slab Heights Versus Elevation Heights

[HumbleChief]

I've been struggling for a long time with a terrain and almost have it figured out but am still a long way from understanding what the software engineers had in mind when they designed the interface.

I think my question illustrates my dilemma.

I have an elevation region defined as -34" and I wanted to creates a concrete patio slab that sits at the same (or slightly above) the terrain. But when I enter -34" as the slab height its much higher than the elevation height with the same value.

Missing something. How does Chief think in this instance?