diff --git a/documentation/ref-manual/ref-classes.xml b/documentation/ref-manual/ref-classes.xml
index bd4f8629da..b6125a5fa1 100644
--- a/documentation/ref-manual/ref-classes.xml
+++ b/documentation/ref-manual/ref-classes.xml
@@ -1976,7 +1976,16 @@
     <para>
         This class inherits the
         <link linkend='ref-classes-package'><filename>package</filename></link>
-        class and is for a given recipe when the recipe inherits the class.
+        class and is enabled through the
+        <link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>
+        variable in the <filename>local.conf</filename> file.
+        <note>
+            You cannot specify the <filename>package_tar</filename> class
+            first using the <filename>PACKAGE_CLASSES</filename> variable.
+            You must use <filename>.deb</filename>,
+            <filename>.ipk</filename>, or <filename>.rpm</filename> file
+            formats for your image or SDK.
+        </note>
     </para>
 </section>