In this transaction you are able to define default document type and posting keys for number of account receivable and account payable transactions. This could help in some cases where business wants to use other document type and in the same time wants to speed up the entry process.
IMG Path: Financial Accounting (New) -> Accounts Receivable and Accounts Payable -> Business Transactions -> Incoming Invoices/Credit Memos -> Make and Check Document Settings -> Define Default Values
When you enter into the transaction, you will see following list with transaction codes and related default document types and posting keys if such are allowed to be defined here.
If you want to make change, just double click on some transaction and you will get following screen where you could maintain the default document type and posting key (not for every transaction you could set default posting key).
Following is a case where you are not able to define default posting key from here.