Default Customer is a setting in Shopify II. If you don't import a Customer or the system doesn't find a match, the Default Customer is assigned to imported Orders.
You might create a Customer you call Shopify Orders for this purpose.
Within an Order you can import all contact information, so it is possible to use one Customer card for all Shopify orders. Or, catch errors where a customer is not found and correct any Orders set to the Default Customer.