Solution : https://service.sap.com/sap/support/notes/181793 (SAP Service marketplace login required)
Key words : 
individual purchase order items, purchase requisitions involved, additional key words, -party order processing, indicator 'fixed date, general confirmation date, complete delivery indicator, individual items, sales order, delivery date
Related Notes : 
       
| 400242 | PReq delivery date with complete delivery: no update | 
| 215611 | PReq delivery date for delivery groups | 
| 214885 | Complete delivery: PREQ delivery date is uncorrelated | 
| 149427 | PReq scheduled for requested delivery date (2) | 
| 139167 | Although Date and qty fixed: PReq scheduled on WLD |