Solution : https://service.sap.com/sap/support/notes/186577 (SAP Service marketplace login required)
Key words : 
milestone billing plan dates, additional key words, final billing dates, sales document consists, final billing date, inactive pricing condition, milestone billing, sales document, billing plan, billing document
Related Notes : 
       
| 179789 | Control PRICING_COPY | 
| 113347 | Down payment process. as of Release 4.0x (standard) |