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.