Re: [webpayments] [Architecture] Payment App is a term used in EMV to describe the applet stored in SC (#31)

@dlongley 
> I disagree that users don't care about payment methods and I definitely think they care about payment instruments. Certainly in existing payment systems they do

I'm not sure if we're on the same page wrt the definition of payment method. In this architecture a payment method is the protocol that is used between payment app and payee to exchange payment data. It defines the format of the payment request and payment response (and possible additional processing the payment app should perform before it can return a response).

When you walk into a store and pick the card you will use to pay do you make that selection based on the messaging protocol that will be used between the POS terminal and your bank? Or based on whether you'll need to put in PIN or signature?

I would assert that you are more interested in who issued the card and what you will get out of using it (loyalty points, rewards, discounts, cash back) or where the funds will come from (credit, bank account, etc).

If you are interested in which payment instrument you use then you'll use payment apps that surface the specific payment instrument as a user choice or use a payment app that only holds a single payment instrument.

> Before we decide to add another abstraction layer, can you list the benefits of doing so?

I don't agree that we are adding a layer of abstraction. We are replacing one model (wallets, payments instruments and payment schemes) with another that is more appropriate to the Web (payments apps, a payment mediator and payment methods).

As I have described in the updated wiki here (https://github.com/w3c/webpayments/wiki/A-Payments-Initiation-Architecture-for-the-Web#definitions), the wallet/payment instrument metaphor doesn't work on the Web.

This model is more appropriate for this architecture as it accommodates the older concepts like physical payment instruments but opens up a far more flexible model for innovation by both the app publishers and PSPs.

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https://github.com/w3c/webpayments/issues/31#issuecomment-163338336