Billing Documents represent your customer's invoices, credit memos and debit memos. Invoices are statements of amounts owed by a customer, and are either generated one-off, or generated periodically from a subscription. They contain invoice items, and proration adjustments that may be caused by changes to a subscription. If your invoice is configured to be charged automatically, Zuora automatically finalizes your invoice and attempts payment otherwise Zuora will email the invoice to your customer and await payment. Any customer credit memos may be applied before determining the amount due for the invoice (i.e., the amount that will be charged).