This action call is used to create a Checkout Session that represents your customer's session as they pay for one-time purchases or subscriptions. It should be used to start the process of collecting card details and creating a customer, to pay for an item or a subscription. Specifically, there are 3 modes that can be used: setup, payment, and subscription. Right now the SCA part of the plugin is built around setup and payment modes.
The App fee parameter is no longer obligatory when sending the Seller ID in the request.