As always, something new to learn, even when it is something old! xsi:type enables overriding the default type of an element with a derived type. See:
Using xsi:type
Using Derived Types in Instance Documents
Another example of how xsi:type is used, in Health IT: Combining time intervals in CDA and some more related discussion on combining time measurements. In terms of Altova XMLSpy support though, my understanding is that with XMLSpy Enterprise Edition 2014, rel. 2 sp1, the Schema tab/view does not indicate that there are derived types available for an element's type, nor does it support switching to view the structure of these derived types. However the Text tab/view does support auto-completing with any of the derived types once we type xsi:type=
