From Halopedia, the Halo wiki
The UA helmet attachment consists of reinforced metal plates or bracers attached to the helmet's forehead to give better protection to the wearer in combat. For more information on other UA attachments, see MJOLNIR Powered Assault Armor/UA.
Two different models are known, one designed for the Military Police helmet, and an Up-Armored variant for the ODST Helmet. The former is standard issue to Special Force unit commanders and Beta-5 Security units patrolling the Inner Colonies, while the latter seems to be exclusively used by the ODSTs. The two devices differ in appearance as well as their pricing.
The External Hardened Uplink or "HUL", is a device compatible with helmet variants of the MJOLNIR Powered Assault Armor. It is designed for acquisition of military intel, and is required for sensitive operations. The cost of one HUL unit is 5,000 credits.