Customer Invoice Templates allow you to define blueprints for what your data should be on your customers' invoices. The key part of a template is what the line items are, what the activity data behind those line items are, and how those line items should be calculated (i.e., charges and price breaks). You use Billing Rules to define how data from the activity tables are used to compute the quantities that appear on each invoice.
In the simplest case, you will do a one-time setup of a Customer Invoice Template in this app, and then, at the end of every billing period, that Template uses its Billing Rules to gather data from the activity tables to generate all of the lines into an Invoice Worksheet.
You may have 1 or more Customer Invoice Templates for each of your customers. You will create these templates when you bring on a new customer, and you'll edit them when you change how you're invoicing a customer, but they will not necessarily change on an ongoing basis unless you make changes to how your customers are to be invoiced.