unit

<xs:attribute name="unit">
  <xs:annotation>
    <xs:documentation>identifies the unit of information conveyed by the element, e.g. columns, pages, volume. Suggested values include: 1] volume; 2] issue; 3] page; 4] line; 5] chapter; 6] part; 7] column</xs:documentation>
  </xs:annotation>
  <xs:simpleType>
    <xs:union>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="volume">
            <xs:annotation>
              <xs:documentation>the element contains a volume number.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="issue">
            <xs:annotation>
              <xs:documentation>the element contains an issue number, or volume and issue numbers.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="page">
            <xs:annotation>
              <xs:documentation>the element contains a page number or page range.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="line">
            <xs:annotation>
              <xs:documentation>the element contains a line number or line range.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="chapter">
            <xs:annotation>
              <xs:documentation>the element contains a chapter indication (number and/or title)</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="part">
            <xs:annotation>
              <xs:documentation>the element identifies a part of a book or collection.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="column">
            <xs:annotation>
              <xs:documentation>the element identifies a column.</xs:documentation>
            </xs:annotation>
          </xs:enumeration>
        </xs:restriction>
      </xs:simpleType>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:pattern value="(\p{L}|\p{N}|\p{P}|\p{S})+"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:union>
  </xs:simpleType>
</xs:attribute>