xsi:type and unions

Hi

I have a question about XML Schema.

If an element E is defined of a type T, and there is a type U derived by
restriction from T, and U is a union whose member types are A, B, and C,
then:

If an instance of element E appears with an xsi:type attribute, will
this attribute be interpreted only as asserting the type of E (e.g., U),
or can it also be used to identify the member type of U (A, B, or C)?

In other words, are all of the following legal in such a case?

<E xsi:type="U">...</E>

<E xsi:type="A">...</E>

<E xsi:type="B">...</E>

<E xsi:type="C">...</E>

Or is only the first of them legal?

I cannot easily find an answer in the Recommendation.

Thanks

Alessandro Triglia