Solution : https://service.sap.com/sap/support/notes/960410 (SAP Service marketplace login required)
Key words : 
reduction parameter 2 = 'reduce forecast values, permit decimal places, forecast values, decimal places, forecast requirements, terms forecast, current period, rhythmic planning, additional information, mrp element
Related Notes : 
       
| 549752 | FAQ: MRP elements in transaction MD04/MD05 | 
| 536270 | Material forecast: Rounding in case of result correction | 
| 528153 | Material forecast: Storing the rounded values |