--- a/model/prov-dm.html	Wed Oct 31 14:45:33 2012 -0600
+++ b/model/prov-dm.html	Thu Nov 01 07:58:33 2012 +0000
@@ -2321,6 +2321,7 @@
 from which they were derived, so that their reliability can be
 investigated.</p>
 
+<p>It is also important to note that a given entity might be a primary source for one entity but not another. It the reason why Primary Source is defined as a relation as opposed to a subtype of Entity.</p>
 
 <p>A <dfn>primary source</dfn> relation is a particular case of <a>derivation</a> of
 secondary materials from their primary sources. It is recognized that
@@ -3930,6 +3931,7 @@
       of derivation is on connecting a generated entity to a used  entity.
 <li> <a href="http://www.w3.org/2011/prov/track/issues/526">ISSUE-526</a>: Added sentence clarifying sentence in section 5.5.2.
 <li> <a href="http://www.w3.org/2011/prov/track/issues/462">ISSUE-462</a>: Added clarification regarding entity attributes (with respect to fixed aspects) and role of identifier with respect to equality.
+<li> <a href="http://www.w3.org/2011/prov/track/issues/518">ISSUE-518</a>: Added clarifying sentence of primary source.
 </ul>
 
 </section>