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 Read more 
Material Account Assignment
Q – When I look at a material movement accounting document, how does one see an example of the determination used in the account assignment?
A – Use config transaction OMWB, select the chart of accounts/or press cancel. Then Select the Simulation button. Enter plant + Material + Movement type to see the account assignment.
