Controls the subscription behavior of the during the initial payment.
"error_if_incomplete": prevent creating the subscription when initial invoice payment fails due to bad payment method or lack of one. Initial invoice which was created in memory to execute the charge will also not be stored
"allow_incomplete": creates the subscription in incomplete status even when the initial invoice payment fails due to the reasons stated above. The subscription is created in "incomplete" status. The customer update the payment method of the subscription and complete the payment within 23hrs or else the subscription will be cancelled.