4.6 xsi:type

One contested feature of XML Schema is an attribute named xsi:type, which can be placed directly on XML instance data elements, even if a pre-existing Schema doesn't permit the attribute.^[1] For existing XML that uses this with simpleTypes, such as those described earlier in this chapter, this is a reasonable course. If the xsi:type identifies a complexType, however, all the problems in the previous section apply. For new development, the less intrusive XForms type model item property should be used, as described at Chapter 5.

^[1] In fact, it's not even possible in XML Schema to define where xsi:type attributes should or should not be allowed.