The customer notification is a part of TAR (Tasks, Actions and Reminders).
The customer notification can help remind customers (for instance) in a friendly way to pay the invoice.
It is possible to send an (default) email to a customer.
With TAR you can help yourself or your employees to get reminders for many things: reminders to pay the invoice, reminders for ends of dates of contracts, and/or other relation, product related services, etc.
Want to know more about TAR?
In that article you can read all about it.