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.
Not using this format for numbering invoices increases the likelihood that invoices will be given numbers that are identical to the numbers on other, unrelated invoices. This can complicate processing by Accounts Payable, and cause delays in vendor payments. Similarly, having duplicate invoice numbers can make reporting, payment details lookup, etc, significantly more challenging.
Please also 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.