Hi,
the notifyAction is called by PayPal to set the payment status after the order is placed in your shop. For example your customer buys something with PayPal but did not pay directly to PayPal, so PayPal sends the payment status open for this order. After a while the customer pays his debts and PayPal sends a request to this action to change the payment status of the order.
Best regards from Schöppingen
Michael Telgmann