Signalled by the handler within its _finalize() method in inc/parser/handler.php, when it finalizes the instruction list. At the time of signaling the instruction list is complete except for two instructions, document_start & document_end
Syntax plugins could register for this event to finalize processing of their data knowing it is complete, e.g. an alternate footnote plugin.
The handler object 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.