diff --git a/doc/guide/classify.xml b/doc/guide/classify.xml
index e843812c88..b1ad6ba70b 100644
--- a/doc/guide/classify.xml
+++ b/doc/guide/classify.xml
@@ -204,6 +204,11 @@ sub-option with code "code" from the DHCPv4 Relay Agent Information option
         expression is allowed in DHCPv4 only.
       </para>
 
+      <para>
+       "relay4" shares the same representation types than "option", for
+       instance "relay4[code].exists".
+      </para>
+
       <para>
         <table frame="all" id="classification-expressions-list">
           <title>List of Classification Expressions</title>