Where does the system determine the Accounting Doc Type?
The Accounting document type is determined in various ways, according to type of transaction, for example the default can be configured in SAP Customizing Implementation Guide–>Financial Accounting–>Accounts Receivable and Accounts Payable–>Business Transactions–>Outgoing Invoices/Credit Memos–>Carry Out and Check Document Settings–>Define Default Values
However in the case where you have a sales billing document and you would like to default a specific accounting document type you should set the value in SAP Customizing Implementation Guide–>Sales and Distribution–>Billing–>Billing Documents–>Define Billing Types
In Field – Document type (V_TVFK-BLART).
If you do not place a default value in this field the system will determine accounting document type RV.