What is the message sent from Skype window when the user lefts the chat or when the chat session is ended

Normally chat will never end when compared to calls. Chat is an ongoing thing. However, it's possible to leave a chat, in which case there are notifications:

Notifications to the person who left chat:

<- CHAT <chat id> MEMBERS username1 username2
<- CHAT <chat id> ACTIVEMEMBERS username1 username2
<- CHAT <chat id> MYSTATUS UNSUBSCRIBED
<- CHAT <chat id> STATUS UNSUBSCRIBED

Notifications to the other people:

<- CHAT <chat id> MEMBERS username1 username2
<- CHAT <chat id> ACTIVEMEMBERS username1 username2

Currently there is not notification when a chat window is closed, but there's a feature request in Jira for that, feel free to add your comments of Vote for it - https://developer.skype.com/jira/browse/SPA-94

Regards,
Mirje

It is true that clsoing the Chat window doesn't make you leave the chat, or end it if noone is left in it.

However, if you're in a multichat (>2 people) the GUI shows an extra option - you can "Leave the chat". Selecting this option will generate an event and notify all other particpants that you have left the chat.

When the 2nd-to-last person leaves, the only person remaining can no longer send messages, and the chat is effectively terminated.

The same functionality is available using the Chat.Leave() method of Skype4COM (even if it's not a multichat) - calling the method sends notifications to everybody left.

(See http://forum.skype.com/index.php?showtopic=82342 for discussion of this...)

To reiterate though, I think that the Leave Chat option should be available for non-multichats as well - I don't see why it's specifically removed from the GUI?!