"Destination" is provided by the client making the subscription request in the POST operation on the subscription collection. Once the subscription is created, you cannot modify the Destination property. If a user needs to change their event destination, they need to DELETE the subscription and then perform a POST operation on the subscription collection with the new "Destination" property.
Log service contains logs that are kept on the device. There is no remote destination since all entries will appear in the log service's log entry collection referenced by the "LogEntries" property.