zGround in ABLconditions,underground case

I use Butterfly to generate a special case, the simulation domain is combined by the sunken space and the space above it.

So it is different from the normal outdoor case(see picture3)

for example, my open sunken space is 4m below the ground, BF will generate an ABLcondition file with zGround = -4.0 ,I wonder if this value is correct to generate the ABLinlet I want, Do I need to set it to 0?
I know we use atmBoundaryLayerInletVelocity as inlet boundary.https://www.openfoam.com/documentation/guides/latest/doc/guide-bcs-inlet-atm-atmBoundaryLayer.html

I would like to know what zGround means and how it is substituted into the formula to calculate the inlet wind speed

Thanks to anyone who noticed this question