Overview
Customer-Specific Invoice Attachments
Specify, by customer, which POD and other customer-facing documents attached to a load are appended to customer invoices.

There are six inclusion actions:
Never Include
Always Include
Always Include POD
Include if Present
Include if Accessorial Present
Include if POD
There is a global configuration that determines which documents are included for customers that do not have specific requirements.
The Carrier Quote and Customer Invoice page layouts display the status of required and optional documents:

Any attempt to email or print an invoice that does not have all the required documents results in a detailed warning message:

The Document Type list is based on the Document Type picklist field on the Customer Invoice Document object. Fields may be added to this picklist or entries can be disabled if they are not used.