Final answer:
Tracing shipping documents to sales invoices and related records checks for the completeness assertion in an audit to ensure all transactions are recorded and prevent understatement of revenues or receivables.
Step-by-step explanation:
When tracing a sample of shipping documents from throughout the year to the details of the sales invoices and to the sales journal and customers' accounts receivable subsidiary ledger, the auditor is testing the completeness assertion. This process is part of substantive testing during an audit where the auditor aims to verify that all transactions that should have been recorded are indeed present in the accounting records. This includes ensuring all sales transactions are reflected in the sales journal, and consequently, in the customers' accounts receivable subsidiary ledger, thus preventing any possible understatement of revenues or receivables.