A door sensor is a set of a magnet and corresponding switch that is installed in the door jamb and the door respectively. These are positioned above the latch and the strike plate.
The magnet engage or disengage the switch when the door is in use. A door sensor is used to fully monitor the state of your door, whether it is open or closed via the Gainsborough Freestyle App. The door sensor kit is sold separately.
You can see the door status as well as your lock status from anywhere through the Gainsborough Freestyle app.