The first column of each row is a row number or an ID number for that link. That number can be entered for a customer/supplier if you are going to configure specific links.
The link, using the ID number from here, is specified using the procedures Update Customer with the parameter Customer-specific EDI Link for INVOIC and the Update Supplier procedure using the parameter Supplier-specific EDI Link for Orders. The parameters can be found under the Extra info button under the Info tab.