Square spring seats (or spring perches) are set onto axle tubes to allow your leaf springs to sit flat on the axle and prevent you from needing to drill the tube of the axle.
Your trailer requires 2 seats per axle.
It is STRONGLY recommended that you do not weld these spring seats onto your galvanized axles. To properly use these seats, you need to place the seat on the body of the axle so the legs of the seat sit on each side of the axle tube. Set your leaf springs on top of these seats and then bolt them securely with a u-bolt kit.