Solution : https://service.sap.com/sap/support/notes/150303 (SAP Service marketplace login required)
Key words : 
additional key words billing document printing, purchase order data, billing category 'p', billing category, billing type ', payment request', payment request, prerequisites, programmed
Related Notes : 
       
| 129815 | Invoice print/EDI:customer order number (BSTKD) | 
| 113347 | Down payment process. as of Release 4.0x (standard) |