Explanation: When parsing an element or attribute that is
based on a union type, the MRM XML parser uses an xsi:type attribute, where
present, to resolve the union. If an xsi:type attribute is
not present, or an attribute is being parsed, the parser tries each union
member type in turn, attempting to cast to the associated simple type, until
the cast succeeds. The order of precedence for the attempted cast is the order
in which the member types are listed in the message model, under the union
type, in the Outline view.
If the data cannot be cast to any of the
simple types within the union, the union cannot be resolved and a parser error
is reported.