At least once
In order to assure delivery of notification-events their send process is based also on retries. Thus, there may exist cases where a notification-event is sent more than once. The notification receiver can deduplicate such occurrences by using the "tag"-property which identifies a notification event uniquely.
No order guarantees
Due to the high-scaling asynchronous processing and the delivery handling no order can be guaranteed for the notification-events of one receiver-recipient.
Optional values
In fact, all values are optional in the implementation of Notification Service validation, so JSON can be empty. From a customer point of view, values marked as 'Optional', are not required to be present.