Which of the following is NOT considered a source document in an accounting system?

In the context of an accounting system, source documents are essential pieces of evidence that reflect a business transaction's details and help in the documentation process. Each category listed in the choices represents a type of document used to support financial recordkeeping, but only one does not function in the same way as a source document.

Bank statements, invoices, and purchase orders serve as foundational pieces for recording transactions directly. Bank statements provide an account of individual deposits and withdrawals, invoices document sales and reflect the amount owed by customers, and purchase orders signify the request to buy goods or services, thereby establishing a liability.

Tax returns, however, are not source documents. Instead, they are derived from the accounting records and summarize financial information for a specified period. Tax returns reflect the outcomes of the financial activities recorded in source documents but do not initiate or provide evidence of transactions themselves. They serve more as a summary tool for tax purposes, rather than as the foundational documentation where transactions are first recorded. Thus, while tax returns are important for compliance and financial reporting, they do not fall into the category of source documents necessary for day-to-day accounting practices.