Single and Multireferenced

A single-referenced accessor does not have an identity (ID) except as a child of its parent.
A multireferenced accessor has an id attribute.

For example

<!--singlereferenced-->
<people>
  <person name="Joe"/>
</people>

<!--multi-reference accessor-->
<people>
  <person name="philip">
    <address href="#address1"/>
  </person>
  <person name="agnes">
    <address href="#address1"/>
  </person>
</people>
<adress id="address1">
  <street>300 Main St.</street>
  <city>Columbia</city>
  <state>South Carolina</state>
</address>

Of course, the hrefs can also be URLs.