XML is still something that often leaves me scratching my head, but you may want to take a look at:

ms-help://MS.VSCC/MS.MSDNVS/cpref/html/frlrfSystemXmlSchemaXmlSchemaEnumerationFacetClassTopic.htm



<?xml version="1.0" encoding="IBM437"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="SizeType">
<xs:restriction base="xs:string">
<xs:enumeration value="Small" />
<xs:enumeration value="Medium" />
<xs:enumeration value="Large" />
 </xs:restriction>
</xs:simpleType>
<xs:element name="Item">
<xs:complexType>
<xs:attribute name="Size" type="SizeType" />
</xs:complexType>
</xs:element>
</xs:schema>