The AP ledger does not specifically require a subsidiary ledger. However, it is recommended for those who want to keep a general ledger organized. This can also help with keeping transactions from getting confused with one another.
A subsidiary ledger is used to contain detailed information regarding transactions that are listed in a summary on a general ledger. This is often used for a high volume account that has multiple transactions and information that is not necessarily needed by a general accountant. The accountant can access the basic information needed for his role, while a financial analyst could use the subsidiary ledger to explore the transactions in greater detail.