An abstract value type that is meant to
be used by the ORB, not the user. Semantically it is treated
as a custom valuetype's implicit base class, although the custom
valutype does not actually inherit it in IDL. The implementer
of a custom value type shall provide an implementation of the
CustomMarshal operations. The manner in which this is done is
specified in the IDL to Java langague mapping. Each custom
marshaled value type shall have its own implementation.