An abstract value type that is meant to be used by the ORB, not the user.

  An abstract value type that is meant to
be used by the ORB, not the user. Semantically it is treated
as a custom value type's implicit base class, although the custom
valuetype 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 langauge mapping. Each custom
marshaled value type shall have its own implementation.