Oracle General Ledger requires at least one set of books (ledger) to be defined for each business location or operating unit. This set of books defines the accounting calendar, currency, and chart of accounts for that location's financial transactions.
In Oracle General Ledger, a Set of Books (ledger) ties together a chart of accounts, calendar, and currency, and every business/legal entity that needs to record transactions must be associated with at least one such set of books. Without one, there would be no structure to post journal entries or generate financial reports for that location.