The event is signalled after Dokuwiki has completed its initialization phase and before it closes the session in doku.php. Most Dokuwiki globals will have been created and populated, the global $_SESSION var will still be in existence.
If you need an even earlier event, check out INIT_LANG_LOAD, but note that not the whole environment is setup there.
Note: Even after $_SESSION has been closed a plugin can reopen the session to access session data - as long as headers have not already been sent. For an example of this refer to lib/plugins/config/admin.php.
No data is passed.
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.