The durasteel underlay was an enhancement made from durasteel that increased the protection capabilities of armor against physical damage, albeit to a lesser degree than the ballistic shielding. The Mark I, II, III, IV, & V versions in turn allowed for minor increases in protection. However, the added safeguards impaired the movement of the wearer.
Behind the scenes
In Star Wars: Knights of the Old Republic II: The Sith Lords, this upgrade can be used on all types of armor, except for robes.