If a vendor invoice has a unique invoice number on it, that should always be used. However, there may be instances in which no such number is available (e.g. honoraria). In such circumstances, please consult this chart to determine the appropriate method for naming the invoice.
Separately, please be advised that invoices numbers featuring spaces or special characters (i.e. not letters or numbers) will be amended by Accounts Payable when they are processed and entered into SAP, in order to prevent issues with SAP that may disrupt payment of the invoice. If you are ever inquiring into the status of an invoice processing/payment and need to reference the invoice number, it can be helpful to know of this potential discrepancy in advance.