Solution : https://service.sap.com/sap/support/notes/382379 (SAP Service marketplace login required)
Key words : 
ps hierarchy reportpsp 1       ---         120          120          120  psp 2     120         ---          ---          120 distrib, cost element-dependent planning data, ps hierarchy reportwbs 1       ---         200          120        200  wbs 2     120         ---          ---        120 case 2, enter cost element planning directly, cost element planning            displayed planning, wbs element +cost element plan, ps hierarchy reportpsp 1       ---         ---          120        0  psp 2     120         ---          ---        120, ps hierarchy reportpsp 1       ---         ---          ---        120  psp 2     120         ---          ---        120, higher-level wbs element, mentions cost element planning
Related Notes : 
       
| 535158 | CJ40, CJ42: Total up function | 
| 397799 | INFORMATION: CO-OM-OPA (Order & Project Accounting) | 
| 390620 | Not possible to total up outside of structure plannings |