If you plan to use multiple Shopify stores and a single accounting system, and want to differentiate between the Invoices or Sales Receipts from these stores, this feature can be used.
Under Orders Settings, while importing Invoices or Sales Receipts from Shopify into QuickBooks Online, you can add a prefix before every Invoice/Sales Receipt number to distinguish between orders from each store.
E.g., If you add a prefix of "ZS", an order in Shopify "#1001" would be imported into QuickBooks Online as Invoice/Sales Receipt "ZS#1001".
Note: You can leave the field empty if you want imported Invoice/Sales Receipt numbers to be identical to the order numbers in Shopify.