The account number is the G/L account you are posting this line item to, such as 113109, a clearing account for incoming payments. If you do not know the number of the G/L account, you can search for it using search help. The system stores certain fields from the G/L account master record in a matchcode. You can search for an account using those fields.

If you want to search for a number, place the cursor in the Account number field. Display the possible entries to get an overview of the available search fields.