Related Rule
United States of America
Practice relating to Rule 66. Non-Hostile Contacts between the Parties to the Conflict
Section C. Definition of parlementaires
The US Field Manual (1956) provides that parlementaires are “agents employed by commanders to go in person within the enemy lines for the purpose of communicating or negotiating openly and directly with the enemy commander”. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 459.
The manual states: “A person is regarded as a parlementaire who has been authorized by one of the belligerents to enter into communication with the other and who advances bearing a white flag.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 460.
Moreover, the manual states: “Parlementaires must be duly authorized in a written instrument signed by the commander of the forces.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 462.