Minecraft wiki entry for Development_versions, associated text: main_hand - An optional object storing optional transform data for first_person and third_person for the player's right hand.
off_hand - An optional object storing optional transform data for first_person and third_person for the player's left hand.
first_person - An optional object storing 3 vectors 'position', 'rotation', 'scale' used to build the first person matrix.
third_person - An optional object storing 3 vectors 'position', 'rotation', 'scale' used to build the third person matrix.