One of the first questions when buying night vision optics is »How far will I be able to see?«
The range of detection is the distance on which you can detect/see a 1.7 m high object.
Always have in mind that there are many different things that influence how far you can see looking through night vision optics. For example, if the object is larger it`s easier to see. Another outside impact is lighting condition, the more light reflecting from the moon, stars (ambient lights), the further and also better you will see. The range of detection is given at average conditions.
The range of detection also applies to digital night vision and thermal imaging devices.