There are 3 designs for the headpiece: 'Melee', or physical attacks, 'Ranged', or ranged/spaced attacks, and 'Magic', or mana consuming attacks. Each of these pieces has different values on it, depending on their respective 'class'.
The armor glows seemingly after being charged by light from any source, even when placed in the social slot, making it very useful as you can wear better armor while still getting the glow from the Mythril Armor. This effect can lead to the brightness of the Orb of Light being magnified to near that of a torch.
The set bonus if the full Mythril Armor set is different based on what head armor you use, and increases the stats based on the 'class' you are.