A message of type PUSH_NOTIFICATION is sent to the system queue when a push notification is received from the server. This message is a notification that messages are queued on the server. It has an expiry of one minute. This expiry time cannot be changed.
This type of system message is useful if you are using the ondemand policy. For example, you can call QAManagerBase triggerSendReceive when a system queue message is received of type PUSH_NOTIFICATION.