Select the number of the account that the entry on the journal line will be posted to.
To see the existing account numbers, click the AssistButton in the field. The list that appears depends on the account type you have chosen in the Account Type field: G/L accounts, bank accounts, customer, vendor or fixed asset accounts.
There are VAT fields associated both to the Account No. and to the Balancing Acc. No. Therefore you should be careful only to define VAT associated to one of the account numbers.