diff --git a/documentation/dev-manual/dev-manual-common-tasks.xml b/documentation/dev-manual/dev-manual-common-tasks.xml
index 0922e62b95..e7d7503e9a 100644
--- a/documentation/dev-manual/dev-manual-common-tasks.xml
+++ b/documentation/dev-manual/dev-manual-common-tasks.xml
@@ -7270,6 +7270,12 @@ Gateways via their Web Interfaces</ulink>"</emphasis>
      EXTERNALSRC = "<replaceable>path</replaceable>"
      EXTERNALSRC_BUILD = "<replaceable>path</replaceable>"
             </literallayout>
+            <note>
+                In order for these settings to take effect, you must globally
+                or locally inherit the
+                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-externalsrc'><filename>externalsrc</filename></ulink>
+                class.
+            </note>
         </para>
 
         <para>