This brake disc is part of the parking brake assembly on the rear axle of the host tractor. It is mounted on the final-drive reduction hub and locks the wheel when the parking brake is engaged. The disc has a splined bore that mates with the hub splines, and the parking-brake caliper clamps the disc to hold the tractor stationary. A worn disc reduces clamping force and can allow the machine to creep on a slope. The manufacturer machines this disc to the same spline profile and thickness as the original component, so no shimming or hub modification is required during replacement.