Solution : https://service.sap.com/sap/support/notes/64734 (SAP Service marketplace login required)
Key words : 
additional key words firming procedure, period lot-sizing procedure, planning time fence lies, planning time fence, material shortage exists, net requirements planning, gross requirements planning, firming procedure, period-end, respective requirements
Related Notes : 
       
| 845181 | Planning time fence doesn't work properly for AFS lot sizes | 
| 201382 | MD19: Selection via planning time fence | 
| 187981 | Planning time fence and periodic lot size | 
| 111708 | Safety stock is available and firming |