NotebookEventActions->{"event1":>action1, "event2":>action2, ...} specifies that actioni should be evaluated whenever "eventi" occurs in connection with the notebook.
The possible event specifications are the same as for EventHandler, with the following addition:
"WindowClose"
window being closed
Settings for NotebookEventActions are often defined at the level of stylesheets, rather than for individual notebooks.
By default, any action specified for an event by NotebookEventActions will override any default action that Mathematica would take for that event.