Hall IC has a hysteresis loop so that the output voltage does not fluctuate immediately even if the magnetic flux density changes a lot.
When the output is high, the output goes low when the magnetic flux density reaches Bop. The magnetic flux density at which the output changes from High to Low is called Bop (Operation Point).
Conversely, the magnetic flux density that changes from Low to High is called Brp (release poimt). Bhyst is the difference between this Bop and Brp.
The phenomenon of chattering is that when a magnetic flux near Bop is applied to a Hall IC, the applied magnetic flux density exceeds or falls below Bop (= Brp) due to the effects of noise, etc., causing the switch to turn ON/OFF in small increments. This is a phenomenon of repeating OFF. Bhyst is wide enough to prevent this chattering. The smaller this Bhyst is, the easier it is to switch the output, and the larger it is, the harder it is to switch.