FieldGroup

<xs:group name="FieldGroup" id="Field_Group">
  <xs:annotation>
    <xs:documentation>Field data will be used to specify field content, formats, requirements...</xs:documentation>
  </xs:annotation>
  <xs:choice>
    <xs:element ref="Field"/>
    <xs:element ref="Fields"/>
  </xs:choice>
</xs:group>