Not all IVES messages are intended for the general public. A given Supplier may be sending an IVES message to a known Consumer who has very specific requirements about the data and how it is classified. The Supplier is therefore entirely within their rights to create a private standard for use purely when sending messages to this particular Consumer. This enables the creation of very specific information structures that can be tailored to a very specific deployment. For example a series of <Description> types could be created for the specific part of the graphic design that is targetted with each document, or a series of <Property> tags would model the specific information hierarchy of the target deployment of the information.
These documents would probably never be publically released, but rather sent directly from Supplier to Consumer , processed and then discarded, and there is no reason that these private types would ever be known outside the specific relationship which they are intended for.