Autodesk Developer Network 2011
Autodesk
Geometry API
§Transformation
§GeometryElement.GetTransformed() - Return a copy of the geometry transformed
§Instance.GetTransform() – return the transform of the instance
§Instance.GetTotalTransform() – return the total transform (e.g., true north transform of an imported instance).
§Serialization/deserialization of Reference
§Reference.ConvertToStableRepresentation() - convert a reference to a stable string representation
§Reference.ParseFromStableRepresentation() – restore the serialized reference. Could be in the same or different Revit session
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public string SerializeReference(Document doc, Reference reference)
{
    String ids = String.Empty;
    ids = reference.ConvertToStableRepresentation(doc);
    TaskDialog.Show("SerializeReference", "Representation:\n" + ids);
    return ids;
}
GeometryElement.GetTransformed() returns a copy of the geometry in the original element, transformed by the input coordinate transformation. The GetTransform() and GetTotalTransformed() methods return the transform of the instance.

We now have the ability to save a reference to a geometry object, say, a face, edge or curve as string and obtain identical reference later using the String as input in the same or different Revit session. So the ConvertToStableRepresentation() converts a reference to a stable string representation and ParseFromStableRepresentation restores the serialized reference.