The <cxxUnionDefinitionFile> element is defined within <cxxUnionAPIItemLocation>. <cxxUnionDefinitionFile> specifies the file in which the API definition is present.
The element <cxxUnionDefinitionFile> is empty but for the required attributes name and value. name is always "filePath" and value is the actual file path.
<cxxUnionDefinitionFile name="filePath" value="C:/epoc32/include/elements/cftransportmsg.h"/>