Re: [webpayments] How are payment instruments registered? (#14)

@ianbjacobs I don't think it's private, it is used by the publisher to probe if their app is installed later on so needs to be known to both the publisher and mediator.

Using a URL has the benefit of enabling some SOP based restrictions. e.g. If the app is installed through some side channel then the mediator is able to determine which origin is able to probe for the presence of the app.

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