Each message type has a list of possible shortodes you can include. Some shortcodes are required because the shortcode is the method used to send you meaningful data from WordPress.
When the Monitor.chat plugin is installed, example messages are included. You can change the examples if you desire. Review the possible shortcodes for each message in the table at the bottom of the page.
The logout message supports all of the emoji shortcodes and two other shortcodes.
The list of Monitor.chat emoji shortcodes can be found here.