Why combining two or more sensing technologies inside one detector reduces both missed alarms and nuisance alarms — instead of just averaging one weakness into another.
A single-sensor detector — a photoelectric smoke detector alone, or an ionization detector alone — makes its alarm decision from exactly one physical measurement. A multi-criteria (sometimes called multi-sensor or combination) detector houses two or more independent sensing elements, such as a photoelectric chamber plus a heat thermistor, or a photoelectric chamber plus a carbon monoxide (CO) cell, and runs an onboard algorithm that weighs all of them together before deciding to alarm. The distinction matters because it changes what a false or missing signal from any one sensor actually does to the system.