The Reinforcement is a Seabase compartment that increases its hull integrity. Each Reinforcement adds 7 units to the Seabase's hull integrity. They are constructed with the Habitat Builder and can be placed on the side panels of Basic Compartments, T Compartments, Moonpools and Multipurpose Rooms.
Interestingly, when a Reinforcement is placed on a corner of a Multipurpose Room, the hull strength is instead raised by fourteen points as opposed to seven. It is currently unknown whether or not this is a bug.