| Marks a property that refers to classes with {@link XmlElement}
or JAXBElement.
Compared to an element property (property with {@link XmlElement}
annotation), a reference property has a different substitution semantics.
When a sub-class is assigned to a property, an element property produces
the same tag name with @xsi:type, whereas a reference property produces
a different tag name (the tag name that's on the the sub-class.)
This annotation can be used with the following annotations:
{@link XmlJavaTypeAdapter}, {@link XmlElementWrapper}. | 