Customer Lines helps track line item details needed in customer procurement systems.
Orders from PO's sent via EDI, cXML, etc. services may contain procurement system data such as line numbers that are replaced by your own but needed when you invoice.
Customer Lines adds Line Item, Line Units, Line Count, Line Number, Text 1, Text 2.
Line Numbers from customer PO's can be added to Orders and included on Invoices. This works for import and export to procurement systems where these are required.