Hysteresis is the difference in the strengths of the magnetic fields when the AMR sensor output turns on and when it turns off.
This is not the difference between Hon and Hoff in the datasheet (example: 0.5 to 2.5mT for the MRMS501A-001).
(Although it varies depending on the individual product, the hysteresis is around 0.1 to 0.4mT. Typically, the size of the hysteresis is not specified in the specifications.)