From Halopedia, the Halo wiki
The casualty classification is a way to determine the status of military personnel. I.E. dead, or missing in action.
List of classifications
Due to the need to increase morale in the UNSC, SPARTAN-II and SPARTAN-III supersoldiers are never listed as 'Killed In Action', but rather as 'Missing In Action' or 'Wounded in Action' to give the appearance that they are invincible.
List of appearances