diff --git a/Forum-Sharing-Client.md b/Forum-Sharing-Client.md
index ba82a28..c23f671 100644
--- a/Forum-Sharing-Client.md
+++ b/Forum-Sharing-Client.md
@@ -18,18 +18,26 @@ The sender sets the forum's visibility to VISIBLE when sending an invite message
 
 **1: ACCEPT** - Sent in response to an invite. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
 
+`previousMessageId` is the ID of the previous message in this session.
+
 The sender sets the forum's visibility to SHARED when sending an accept message. The recipient sets the forum's visibility to SHARED when receiving an accept message.
 
 **2: DECLINE** - Sent in response to an invite. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
 
+`previousMessageId` is the ID of the previous message in this session.
+
 The recipient sets the forum's visibility to INVISIBLE when receiving a decline message.
 
 **3: LEAVE** - Sent by either party when unsubscribing from the forum, if the forum's visibility is VISIBLE or SHARED. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
 
+`previousMessageId` is the ID of the previous message in this session.
+
 The sender sets the forum's visibility to INVISIBLE when sending a leave message. The recipient sets the forum's visibility to INVISIBLE when receiving a leave message.
 
 **4: ABORT** - Sent by either party when recieving a message that is valid but unexpected in the current state. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
 
+`previousMessageId` is the ID of the previous message in this session.
+
 The sender sets the forum's visibility to INVISIBLE when sending an abort message. The recipient sets the forum's visibility to INVISIBLE when receiving an abort message.
 
 ### State machine