A geographic transformation is used when projecting geometries between two different geographic coordinate systems. A geographic transformation converts everything that needs to be changed including the units, prime meridian, and the ellipsoid. Every transformation is defined in a particular forward direction, say from GCS A to GCS B, but all are reversible. For example, a geographic transformation may be defined to convert from NAD27 to WGS84. If you are projecting from WGS84 to NAD27, you can use the reversed form of the transformation.

public sealed class GeographicTransformation : DatumTransformation 

Public NotInheritable Class GeographicTransformation 
   Inherits DatumTransformation