Final answer:
An auditor tracing sales transactions back to orders and shipping documents is testing the occurrence assertion, verifying that sales actually took place and are properly recorded.
Step-by-step explanation:
When an auditor traces a sample of sales transactions recorded in the sales journal back to the customer orders and shipping documents, they are testing the occurrence assertion. This assertion concerns whether the sales transactions actually took place and are valid. By checking that each recorded sale has a corresponding customer order and a shipping document, the auditor verifies that the transactions happened and were properly authorized, thus confirming the existence of the sales recorded in the sales journal.