Solution : https://service.sap.com/sap/support/notes/158012 (SAP Service marketplace login required)
Key words : 
additional key words cs20, transaction cs20 delivered prior, batch input data, item category l', batch input session, 'corrections' -> 'correction instructions', batch input processing, multiple/variant boms, component unit 'part', 'add document' function
Related Notes : 
       
| 159321 | Mass change for Add BOM item | 
| 138981 | Bills of material: Restriction mass change | 
| 128934 | Error in BOM group maintenance | 
| 121144 | |
| 116148 | Error in BOM group maintenance |