Final answer:
In a firm with good internal controls, credit approval is indicated on the sales order. It is a preventive measure against unauthorized transactions and helps maintain separation of duties.
Step-by-step explanation:
In a firm that practices good internal controls within the sales and collections cycle, the function of indicating credit approval should be documented on the sales order. This ensures a separation of duties and helps to prevent unauthorized transactions. The sales order is the initial document that signals the agreement between the customer and company, marking a good point to verify and record credit approval before the sales process continues.