For most of the automatic emails in Books, I understand when they are generated. But for instance, what triggers the "Customer Notification" email? What does a customer have to do, specifically, to trigger the "Payment Initiated" email?

Also, does the %User% placeholder work for the User and Client Portal Invitation emails - does it capture the current user when it generates that email? Same question for the Customer Notification, Client Review, and Request Payment Information emails?

Thank you!