When you add a customer or when a customer signs up through the portal on Class Manager, you or them will have to enrol their student into a class.
When a student has been added to a completed class, the system will know that they need an invoice. So this can mean: a) if you as the user have manually attached the student; or b) if you have online enrolments activated and they have enrolled with that method.
If a customer has already had an invoice created for them for that particular class, the system will know that they have been billed for it and it will not create another invoice for them.