--- a/model/optional.html	Fri Mar 30 12:26:50 2012 +0100
+++ b/model/optional.html	Fri Mar 30 13:43:45 2012 +0100
@@ -26,6 +26,7 @@
 </ol>
 
 <h4>Optional derivation-related</h4>
+If some of the following attributes are not specified, the derivation path is not specified. It is always possible to infer some activity, generation and usage, for which the derivation holds (though not necessarily in current account if constraint on generation uniqueness holds).
 <ol>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#derivation.activity">activity</a>
 <li><a class='attribute' href="http://dvcs.w3.org/hg/prov/raw-file/default/model/prov-dm.html#derivation.generation">generation</a>