Mandatory check box on the Purchases & Payables Setup window?

When processing purchase invoices, employees at a company enter the following
information: order number, invoice number, vendor number, vendor order number, vendor
invoice number, and item numbers. Which of these numbers can be made mandatory by
using the Ext. Doc. No. Mandatory check box on the Purchases & Payables Setup window?

A.
Vendor invoice number

B.
Vendor number

C.
Vendor order number

D.
Invoice document number