The xs:QName Type
SQL Server does not support types derived from xs:QName by the use of an XML Schema restriction element. Also, SQL Server currently does not support union types with QName as a member type.
Example
The following CREATE XML SCHEMA COLLECTION statements cannot load the XML schema, because they specify the xs:QName type as a member type of the union:
CREATE XML SCHEMA COLLECTION QNameLimitation1 AS N'
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="myUnion">
<xs:union memberTypes="xs:int xs:QName"/>
</xs:simpleType>
</xs:schema>'
GO
CREATE XML SCHEMA COLLECTION QNameLimitation2 AS N'
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="myUnion">
<xs:union memberTypes="xs:integer">
<xs:simpleType>
<xs:list itemType="xs:QName"/>
</xs:simpleType>
</xs:union>
</xs:simpleType>
</xs:schema>'
GO
Both statements fail with an error.