Index: book.xml
===================================================================
--- book.xml	(revision 41237)
+++ book.xml	(working copy)
@@ -3808,6 +3808,11 @@
 
 	<para>For the complete list of such values, please see <xref
 	    linkend="uses-values"/>.</para>
+
+	<warning>
+	  <para><makevar>USES</makevar> cannot be assigned after inclusion of
+	  <filename>bsd.port.pre.mk</filename>.</para>
+	</warning>
       </sect2>
 
       <sect2 id="use-vars">