Solution : https://service.sap.com/sap/support/notes/91109 (SAP Service marketplace login required)
Summary : 
      
Key words : 
vat registration number reason, terms vat registration number, vat registration number, prerequisites standard behavior, standard system behavior, attached program solution, standard behavior, required behavior, program v05ezzrg, program v05ezzag
Related Notes : 
       
| 497418 | M/SD: UID of the payer is missing in the billing doc. | 
| 434562 | Determination and country of VAT registration number | 
| 148505 | |
| 119680 | |
| 86358 | An incorrect value-added tax is displayed | 
| 21734 | Determining sales tax (VAT) from any partner | 
| 15244 | Incorrect VAT amount is displayed |