The hysteresis indicates a tolerance range outside the geofence radius. This essentially is a threshold the vessel will be required to exit prior to alerting your Siren Marine app. The GPS built into the unit may not provide the exact coordinates of the vessel (ex. 1-5 meters off point) due to interfering factors, possibly resulting in the vessel temporarily exiting the geofence radius (this will also depend on how small the radius has been configured). This is why the hysteresis is set in the event the vessel exits the radius.
The alert will be triggered as follows:
Exiting Geofence: Exit Radius > Hysteresis > Alert Delay > Alerts App
Entering Geofence: Enter Radius > Alerts App