Re: [w3c/webpayments] Payment Methods vs Payment Instruments (#124)

> I feel that section 4.1 (Supported vs Enabled Payment Methods) in the Payment Apps spec proposal has some room for improvement.

Based on your comment I would agree but only because I think you have misunderstood the intention of payment method.

Our architecture intentionally steers clear of the whole concept of payment instruments. Instead users have payment apps which may simply be a wrapper around a payment instrument or may be something completely different.

A payment method is simply a description of the data that is required in a payment request and the data that is required in the payment response. Implicit in that is the processing of the request that is required to produce an appropriate response.

"Visa" and "MasterCard" are actually poor examples of payment methods as they are really just basic card payments. (i.e. The response should contain a PAN, expiry CVV etc). The reason we have different identifiers for them is to allow merchants to be explicit about WHICH cards they accept because if we just had a payment method called "cards" then that would imply that all merchants accept all cards.

The difference between supported and enabled methods is a difference between the design of the app and the configuration. If I wrote payment app that allowed users to store their credit card details in the app and then send these securely in the payment response I could say that the app *supports* all of the card payment methods. However, until the user actually stores a card, none of these are *enabled*. 

My app might also *support* payment via Bitcoin but until the user configures the app to have access to a Bitcoin wallet from which it can send payments that payment method is not *enabled*. To take that further, my app might support 3 different ways of paying by Bitcoin defined as 3 different payment methods. In that case, as soon as my app has access to a Bitcoin wallet to make payments all 3 payment methods become enabled.

**TL;DR:** There is NOT a strong link between a stored payment instrument and an enabled payment method. Storing a payment instrument MAY result in one or more payment methods being enabled but that is entirely dependent on the payment method and how the app is designed.

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