The indoor air quality sensor that Swidget uses in its Air Quality Insert (https://www.swidget.com/products/wi-fi-control-air-quality) is a relative sensor. It estimates indoor air quality based on its previous exposure to the air in your home. As the sensor is subjected to more variations in air quality, like good air from the outdoors or poor air from cooking, it gets better at estimating your indoor air quality.
How does it compare to an absolute sensor? An absolute sensor's measurements are based on a set reference point and a relative sensor's readings are based on the air quality it's exposed to.