This number is stored in map mainly for exports to other vector formats (e.g ShapeFiles) A contour map that's exported to a shape file will use this value as a Z-value attributes for its shapes.

public void ZValue(
	double val
)

Public Sub ZValue ( 
	val As Double
)

public:
void ZValue(
	double val
)

member ZValue : 
        val : float -> unit