The schema for XML Schema then uses them to define the 'schema' element.

The snippet of that declaration:

<xs:element name="schema" id="schema">
   <xs:complexType>
      <xs:complexContent>
        <xs:extension base="xs:openAttrs">
          <xs:sequence>
            <xs:choice minOccurs="0" maxOccurs="unbounded">
              <xs:element ref="xs:include"/>
              <xs:element ref="xs:import"/>
              <xs:element ref="xs:redefine"/>
              <xs:element ref="xs:annotation"/>
            </xs:choice>
            <xs:sequence minOccurs="0" maxOccurs="unbounded">
              <xs:group ref="xs:schemaTop"/>
              <xs:element ref="xs:annotation" minOccurs="0"
                          maxOccurs="unbounded"/>
            </xs:sequence>
          </xs:sequence>
        ...
        </xs:extension>
      </xs:complexContent>
   </xs:complexType>
</xs:element>