Solution : https://service.sap.com/sap/support/notes/330298 (SAP Service marketplace login required)
Key words : 
short            10            apo transfmedium          15            apo sched transflong            20            apo sched, sap standard production program, enter data type 'char', short text enter 'indicator, symptom backorder processing, /3 customer order items, sourcing main items, additional key words, sourcing main item, dictionary structure vbbed
Related Notes : 
       
| 495337 | Creating/changing delivery with immediate goods issue (APO) | 
| 375193 | Backorder processing: Composite SAP note on posting | 
| 365026 | Composite SAP note: SAP Best Practices Automotive 4.6CV1 | 
| 351428 | Integration APO <--> R/3, sales order, sub-items | 
| 332416 | Main items not relevant for requirements in APO | 
| 321913 | Transfer of the backorder processing results | 
| 321909 | Structure - results of backorder processing |