The subsidiary ledger allows for tracking transactions within the controlling account in more detail.
Individual transactions are posted both to the controlling account and the corresponding subsidiary ledger, and the totals for both are compared when preparing a trial balance to ensure accuracy.
Thus, while the "accounts receivable balance" can report how much the company is owed, the accounts receivable subsidiary ledger can report how much is owed from each credit customer.
[2] In common use, control accounts refer to those that would, under ideal circumstances, balance to zero.
If the discrepancy is significant, then actions such as stock counts can be triggered in order to validate stock and correct the balance sheet and clear the control account.