enum RecurringProcessingModel
How a stored payment method may be used for a later payment, following the card networks' stored-credential framework. The model is declared when a method is stored and again on every payment made from it, and it decides how that later payment is treated for Strong Customer Authentication: a Subscription or UnscheduledCardOnFile charge is one you initiate and falls outside SCA, while a CardOnFile charge does not.
What separates the two models you initiate is the schedule, not the amount. A run of payments on a fixed interval is a Subscription even when the amount differs every time.
CardOnFile— Details kept so a returning customer checks out faster, where the later payment is one the customer starts themselves — a "pay with my saved card" button in your own checkout, for example.Subscription— You initiate the payment, and the payments follow a fixed schedule the customer agreed to in advance. The amount may be fixed or may vary from one charge to the next: a monthly invoice whose total changes every month is still a subscription, because it is the interval that is fixed.UnscheduledCardOnFile— You initiate the payment, and the payments follow no fixed schedule. For example a top-up triggered by a balance falling below a threshold, or an invoice raised whenever a job is finished.