The new paymentSessionModal mutation raises a modal that will be displayed to the buyer after purchase. This modal uses provided data to display additional info to buyers. For example, you could show a QR Code that a buyer can scan, or render message telling buyers to complete an action on an external device.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/new-mutation-to-add-post-pay-actions-onsite