Rather than using xs:ID, we can duplicate what ID does without restricting the value to xs:NAME.

Suppose we have a description of a date and references to that date in some historical text.

We can make the type xs:date and guarantee that there is only on description for each date:

<xs:element name="description-of-date">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="xhtml:p" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="date" type="xs:date" use="required"/>
  <xs:complexType>
  <xs:unique name="date-is-unique">
    <xs:selector xpath="."/>
    <xs:field xpath="@date"/>
  </xs:unique>
</xs:element>