Vendor Discount Code

There is a field Discount Code on AP10.1, under the "Defaults" Tab.
This is what the help text says -

Type or select a valid discount code for the vendor. Used to default a general ledger discount account to the vendor's invoices during invoice entry. If left blank, the vendor uses the discount code assigned to the process level or company.

If assigned, the code defaults on all invoices for the vendor unless you override it for one or more companies in the Vendor - Company Codes subform or override it during invoice entry.