Companies often has different signing limits for vendor invoices related to a puchase order than cost invoice which often are more personal expense.

Unfortunately you can only select "Invoice" when you set iup signing limit policy under Organization administration > Setup > Signing limits > Signing limit pilicies - default signing limits.

The idea is to have two document types for invoice:

1. PO Invoice (must have a reference to a PO)
2. Cost invoice (don't have a reference to a PO - use procurement category on the pending vendor invoice line