usage 

specifies the optionality of the attribute.

Attribute Information

Used By

Element tei:attDef

Source 

<xs:attribute name="usage" default="opt">
  <xs:annotation>
    <xs:documentation>specifies the optionality of the attribute.</xs:documentation>
  </xs:annotation>
  <xs:simpleType>
    <xs:restriction base="xs:token">
      <xs:enumeration value="req">
        <xs:annotation>
          <xs:documentation>(required)</xs:documentation>
        </xs:annotation>
      </xs:enumeration>
      <xs:enumeration value="rec">
        <xs:annotation>
          <xs:documentation>(recommended )</xs:documentation>
        </xs:annotation>
      </xs:enumeration>
      <xs:enumeration value="opt">
        <xs:annotation>
          <xs:documentation>(optional )</xs:documentation>
        </xs:annotation>
      </xs:enumeration>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>

Sample