diff --git a/tangostationcontrol/tangostationcontrol/devices/sdp/packet.py b/tangostationcontrol/tangostationcontrol/devices/sdp/packet.py
index ae21c8f5ac187aa413274a0500ff874eed23b5c6..611017659302f0cd0e346e2c3265e002bc52faf8 100644
--- a/tangostationcontrol/tangostationcontrol/devices/sdp/packet.py
+++ b/tangostationcontrol/tangostationcontrol/devices/sdp/packet.py
@@ -73,7 +73,12 @@ class SDPPacket(object):
 
     @classmethod
     def valid_markers(cls):
-        """ Valid values for the 'marker_raw' header field for this class. """
+        """ Valid values for the 'marker_raw' header field for this class. 
+
+            Each new packet class that introduces a new marker should be added
+            to the PACKET_CLASS_FOR_MARKER registry, which holds the mapping
+            marker -> packet class.
+        """
 
         # return all markers registered in PACKET_CLASS_FOR_MARKER which are for this class or any of its specialisations
         return [marker for marker, klass in PACKET_CLASS_FOR_MARKER.items() if issubclass(klass, cls)]