It has a unique legendary effect which provides a Damage Resistance and Energy Resistance of 15 at 0 Rads, in addition to the resistance provided by the armor. They both increase linearly according to the following: for every 100 Rads the player character currently has, their total DR and ER will both increase by 5. Specifically, if 10% of their health bar is taken by radiation, they will gain 5 DR and ER. If 90% of their health bar is taken by radiation (900 Rads), they will gain 45 DR and ER for a total of 60.
The specific form of this item depends on the player character's level: below level 40, it spawns as either a left or right arm; above level 40, it spawns as a chest piece. Note that these items are generated dynamically, as their Form ID starts with "FF".