Explain how the SAP system records the accounting entries into the general ledger for accounts receivable (which gets the direct posting, which gets the indirect posting, how they stay in balance).

The SAP system records the accounting entries directly into individual customer accounts in the Accounts Receivable sub-ledger of the General Ledger and records the transactions indirectly into the Accounts Receivable Control Account in the General Ledger.

As each entry posted to an individual customer's account is also posted to the Accounts Receivable Control Account on the same side, the two accounts are always in balance.  The corresponding entries are made in the right General Ledger account once.  

When balances are extracted, the sub-ledger Accounts Receivable for individual customers are not extracted, but the Control Account balance is extracted for the Trial Balance.

Explanation:

As an integral part of sales management, the SAP maintains an Accounts Receivable sub-ledger for recording and managing accounting data of all customers.  The postings in the Accounts Receivable component are also recorded indirectly in the Accounts Receivable Control Account in the General Ledger.