From Halopedia, the Halo wiki
UA, short for up-armor, is a series 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.
Halo 3 MCC
All UA variants of armor types can be unlocked in the Halo: Reach's Armory through the game's purchase system via the use of cR. In Halo: The Master Chief Collection, the UA variants of armor types are attainable in the Season 1: Noble progression system.
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.
Halo 5: Guardians
The UA variant of the Grenadier helmet, the Collar/Grenadier [UA] chest piece, and the UA/Bracer wrist attachments are built into the INDOMITABLE-class Mjolnir, unlockable in armor customization through the Requisition system.
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