id function and xsi:type

In http://www.w3.org/TR/xforms11/#fn-id it says

 An element node can be assigned an ID by means of an xml:id attribute or an
 attribute that is assigned the type ID by a DTD or xsd:ID or any type
derived
 from xsd:ID by an XML schema, an xsi:type attribute, or the type model item
 property.

I do not see how xsi:type can be used to assign type xsd:ID to an
attribute, since it appears that xsi:type can assign types only to
elements.

Leigh.