Hoff is the strength of the magnetic field required to switch off the AMR sensor.
The figure in the datasheet shows the required magnetic field in order to switch off the AMR sensor under fixed conditions.
The Hon and Hoff values have a difference of about 1 to 2mT, but AMR sensor products have variations in sensitivity. The sensitivity of each individual AMR sensor is predetermined, but when the products are mass produced, sensors that turn on and off are mixed between Hon and Hoff.