In the import customer options, you can choose how many customers will be imported each AJAX request.
1. Customer per ajax request: Import a number of customer data per Ajax request.
2. Import customers within x day(s): Import only the customer data from the past x day(s).
For instance, setting “Import customers within 30 days” implies that you wish to import only those customers from Shopify who have been added in the last 30 days.
3. Customer role: Set role for imported customers
4. With Purchase(s) only: Only import customers who have at least 1 purchase.
5. Update existing customers: If enabled, the customer will be check if existed by their Shopify ID