The ABS sensor mounting bolt holds the sensor in place at the wheel. It must hold the sensor at an exact distance from the tone ring. Since it is located behind the brake rotor, it gets hot and wet. These bolts frequently rust and snap when you try to remove them.
Applying penetrating oil before loosening the bolt is recommended. If the bolt snaps, you must drill out the thread. Applying anti-seize to the new bolt prevents future headaches.