Today I worked out how to restrict the values for a drop down box via the XML Schema Definition for a form, very handy.
See the following schema extract :

 

        <xs:element name="Breakfast">
          <xs:simpleType>
            <xs:restriction base="xs:string">
              <xs:enumeration value="Bacon" />
              <xs:enumeration value="Eggs" />
              <xs:enumeration value="Sausages" />
              <xs:enumeration value="Beans" />
              <xs:enumeration value="Toast" />
              <xs:enumeration value=""/>
            </xs:restriction>
          </xs:simpleType>
        </xs:element>