diff --git a/include/libwebsockets/lws-smd.h b/include/libwebsockets/lws-smd.h
index 66484d734..22992540c 100644
--- a/include/libwebsockets/lws-smd.h
+++ b/include/libwebsockets/lws-smd.h
@@ -52,6 +52,8 @@ enum {
 	 * Something happened on the network, eg, link-up or DHCP, or captive
 	 * portal state update
 	 */
+
+	LWSSMDCL_USER_BASE_BITNUM				= 24
 };
 
 /**
diff --git a/lib/system/smd/README.md b/lib/system/smd/README.md
index c61ef4527..34ef43f25 100644
--- a/lib/system/smd/README.md
+++ b/lib/system/smd/README.md
@@ -44,13 +44,18 @@ appear as soon as a participant appears that wants them.  The message generation
 action should be bypassed without error in the case lws_smd_msg_alloc()
 returns NULL.
 
+Various well-known high level classes are defined but also a bit index
+`LWSSMDCL_USER_BASE_BITNUM`, which can be used by user code to define up to 8
+private classes, with class bit values `(1 << LWSSMDCL_USER_BASE_BITNUM)` thru
+`(1 << (LWSSMDCL_USER_BASE_BITNUM + 7))`
+
 ## Messaging guarantees
 
 Sent messages are delivered to all registered participants whose class mask
 indicates they want it, including the sender.  The send apis are threadsafe.
 
 Locally-delivered message delivery callbacks occur from lws event loop thread
-context 0 (the only one in the default case LWS_MAX_SMP = 1).  Clients in
+context 0 (the only one in the default case `LWS_MAX_SMP` = 1).  Clients in
 different processes receive callbacks from the thread context of their UDS
 networking thread.