Fires when a system message is received. System messages are a catch-all category for messages received in via the chat system. Examples:
The server message which appears upon login (e.g. "Welcome to Patch 3.4! If you encounter interface problems, please disable your addons and delete your WTF folder.")
The feedback message which appears when the player enters AFK or DND status
Results from a /who query, if the query has three or fewer results
Notification that a friend or guild member has logged in or gone offline
Many standard system message patterns can be found as localized format strings in FrameXMLGlobalStrings.lua (after using the AddOn Kit to extract the default interface files). When this event is received, the message has already been localized and formatted, but using the format string may be useful for parsing variables from the message.