Under the Settings, there is an option to matches the Shopify Customers to the Intacct customers using some unique key. The integration will then automatically start mapping the customer using it.
There is an option to enable the creation of a New Customer in Intacct via the integration in case the unique key of the customer from Shopify to Intacct doesn’t match.
But when this option to create new customers is disabled then, you’ll have a different option to manually map the customers between systems if the unique key is not getting matched or if you want to edit the mapping as per your requirements.
The Customer mapping page will allow you to map & edit the mapping of the customers.