A hv (horizontal/vertical) datum transformation is used when projecting z-aware geometries between two different geographic coordinate systems and two different vertical coordinate systems. As with geographic transformations, a hv datum transformation converts everything that needs to be changed including the units, prime meridian, and the ellipsoid. In addition, a hv datum transformation transforms height. Every transformation is defined in a particular forward direction, but all are reversible. For example, suppose your z-aware geometry is in WGS84 with vertical coordinate system EGM2008_Geoid, and you want to project to NAD83_2011 with vertical coordinate system NAD83_2011. A hv datum transformation is defined to project your z-aware geometry from WGS84 with vertical coordinate system EGM2008_Geoid to NAD83_2011 with vertical coordinate system NAD83_2011. If you are projecting from NAD83_2011 with vertical coordinate system NAD83_2011 to WGS84 with vertical coordinate system EGM2008_Geoid to NAD83_2011, you can use the reversed form of the transformation.

public sealed class HVDatumTransformation : DatumTransformation 

Public NotInheritable Class HVDatumTransformation 
   Inherits DatumTransformation