Am I right in thinking that datatyping at element level ie <xs:element
name="num" type="xs:integer">

and specifying a choice of attribute values ie 

<xs:attribute name="kind">
<xs:simpleType>
          <xs:restriction base="xs:token">
            <xs:enumeration value="volume_number"/>
            <xs:enumeration value="page_range"/>
          </xs:restriction>
</xs:simpleType>
</xs:attribute>

......does not go together??????

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Setting out to convert the following DTD element specification... 
<!ELEMENT num (#PCDATA | emph)*>
<!ATTLIST num
kind (volume_number | page_range) #REQUIRED>

... into Schema....
<xs:element name="num">
    <xs:complexType mixed="true">
      <xs:sequence>
        <xs:element minOccurs="0" maxOccurs="unbounded" ref="emph"/>
      </xs:sequence>
              <xs:attribute name="kind" use="required">
        <xs:simpleType>
          <xs:restriction base="xs:token">
            <xs:enumeration value="volume_number"/>
            <xs:enumeration value="page_range"/>
          </xs:restriction>
        </xs:simpleType>
      </xs:attribute>
    </xs:complexType>
  </xs:element>

.... I realised, that if I want to make use of the additional
datatyping capabilites of Schema (ie adding a datatype of xs:integer
to the element "num" to allow numerical element content only, for
example), I cannot at the same time specify a choice of required
attribute values, i.e. volume_number and page_range for this element. 
As soon as I give the element a datatype
ie <xs:element name="num" type="xs:integer"> no further child element
or attribute specifications are allowed...

I know I can set the integer at the attribute level, which means that
the data needs to be  entered within the <num ...."31"> tag rather
than between an opening and closing
<num kind="volume_number">31</num>
but the element is part of a digitisation project template for
transcribers and the idea is, to set up the template in such a way
that it can check data entry at element level, ie keyers only need to
enter data between element tags, not at attribute level....

I guess I am stuck with adding child elements to the <num> element ie
<volume_number> <page_range>  and add some datatyping to these...???