Final answer:
The auditor is testing the completeness assertion.
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.
The completeness assertion is one of the five main assertions that auditors use to evaluate the accuracy and completeness of financial statements. It focuses on whether all relevant transactions and information have been recorded and included in the financial statements.
In this specific case, the auditor is tracing shipping documents to sales invoices, sales journal, and customers' accounts receivable subsidiary ledger to verify that all sales transactions have been appropriately recorded and accounted for.