Solution : https://service.sap.com/sap/support/notes/453538 (SAP Service marketplace login required)
Key words : 
additional key words transaction /sapapo/bop, backorder processing sales orders, rule-based atp, /sapapo/bopi, open quantity, solution note 441932, prerequisites, transferred
Related Notes : 
       
| 455860 | Backorder processing: ATP check in the past | 
| 375193 | Backorder processing: Composite SAP note on posting |