The webhook can return custom properties from the payment provider as an additionalProperties object in the response. This data is saved with the payment group for the order. The webhook can also return a customPaymentProperties object that specifies a list of the properties in the additionalProperties object that should be returned to the storefront in the response to the original createOrder request. For example:

{
     "orderId": "o60412",
     "paymentId": "pg60411",
     "siteId": "siteUS",
     "merchantTransactionId": "324a5107-8fe5-4dd7-aa1f-8b7e2e0ec8df",
     "hostTransactionId": "o60412-pg60411-1465342612829",
     "transactionTimestamp": "2016-06-07T23:36:52+0000",
     "hostTimestamp": "2016-06-07T23:36:52+0000",
     "transactionType": "0100",
     "additionalProperties": {
          "interestRate": "0.05",
          "remainingPayments": "5",
          "latePayment": false,
      },
     "customPaymentProperties": ["remainingPayments", "latePayment"],
     "amount": "000000007700",
     "currencyCode": "USD",
...