<element name=“root” xmlns=“relaxng.org/ns/structure/1.0”>

<choice>
  <element name="child">
    <attribute name="type">
      <value>Foo</value>
    </attribute>
    <!-- Define stuff that's only valid when type is "Foo" -->
  </element>
  <element name="child">
    <attribute name="type">
      <value>Bar</value>
    </attribute>
    <!-- Define stuff that's only valid when type is "Bar" -->
  </element>
</choice>

</element>