Hull plating relays were a technology integral to polarizing the hull plating of 22nd century Starfleet starships.
In 2154, while fighting a version of Enterprise NX-01 from the future, the present-day Enterprise lost its ability to polarize the hull plating when Lorian, commander of the future Enterprise, ordered his tactical officer to fire on the hull plating relays of their counterpart. (ENT: "E²")