By including this parameter, you're indicating your intention to use this PaymentRequest's payment method for future payments. When successful, providing this parameter will link the payment method to the PaymentRequest's Customer (if available). If no Customer was provided initially, the payment method can still be linked to a Customer after the transaction. Choose between: off_session: If the customer's presence in the checkout flow is uncertain. on_session: If you only plan to reuse the payment method when the customer is present during checkout.