From Halopedia, the Halo wiki
The X-24 FAV was equipped with two machine guns for defense, and armor plating capable of largely negating weapons fire from both bullets and plasma.
The X-24 FAV was not named in the initial Halo: Collateral Damage release, but was first identified in the comic's hardcover release. The APC is only identified as the "X-24 FAV", leaving the acronym FAV unknown. Given the existence of the M12 Force Application Vehicle, it is possible the X-24 FAV stands for Force Application Vehicle.