Default: Default WooCommerce Order method.
All Orders which content only Virtual Products: Product which doesn’t have any other physical products and only have Virtual product will mark as completed.
All Orders which have Paid Successfully: All orders (along with physical products) which successfully paid the order payment will mark as completed.
All Orders: No matter what is the order content or has (payment complete, virtual, physical, offline payments, etc.) as soon as a customer land on the thank you page the order status will be changed to complete.