This object represents an Invoice in API context.
| || ||A unique external identifier for the customer that is used to associate invoice with customer.|
| || ||A unique external reference to the invoice (only for ad hoc invoices).|
| || ||The unique invoice number (only when the invoice is approved).|
| || ||The date when the invoice was drafted. For Ad-hoc invoices it is specified by the user, for period invoices it is set to BillingPeriodEnd.|
| || ||The start date of the billing period.|
| || ||The end date of the billing period.|