Military messengers are a class of royal servants dedicated to delivering the Fire Nation Royal Family's messages. They are not restricted to the palace when undertaking their tasks; as the name suggests, these messengers often carry classified letters to highly ranked military officials. They are usually put to work when the more commonly employed messenger hawks would be unable to adequately carry out a mission.
A military messenger was sent for Prince Zuko to inform him that a war meeting was taking place. He said that the High Admirals, High Generals, War Ministers, and the princess had all arrived and were waiting for him so that they could begin.