Solution : https://service.sap.com/sap/support/notes/570056 (SAP Service marketplace login required)
Key words : 
attached source code corrections, 'rules-based atp' indicator, 'start immediately' indicator, 'process parameter' tab, connected oltp systems, confirmed quantity reason, 'check parameters' tab, check instructions, backorder processing, rule evaluation'
Related Notes : 
       
| 585988 | Confirmed quantity of the main item is not updated | 
| 567291 | Termination during BOP with postprocessing incorrect data | 
| 375193 | Backorder processing: Composite SAP note on posting |