From Halopedia, the Halo wiki
UA, short for up-armor, is a category of supplemental armor attachments and accessories compatible with the MJOLNIR Powered Assault Armor. Like some other MJOLNIR variants, many of the UA pieces are not exclusive to the MJOLNIR system, but are in use by regular UNSC Army or Marine troops as well.
Most of the helmet variants available in Halo: Reach have an up-armored upgrade, sometimes coupled with other external helmet additions such as the UA/HUL attachment to the Mark V [B] helmet. UA helmet attachments consist of reinforced metal plates or bracers attached to the helmet's forehead to give better protection to the wearer in combat.
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