The CRM invoice is the standard invoice that comes with Dynamics 365. We have added an additional form to display more information such as the paid amount and the amount remaining field. The Sales Document is another entity that resides within the Recurring Billing product. The Sales Document allows you to create multiple document types such as Renewals, Quotes, Invoices or any other document type you want to create. This would allow you to use different document types and report on this information. During the setup, you choose which you want to use, either the CRM Invoice or the Sales Document. You would not use both.