This is Active Infrared Detector. Install each pair at the opposite positions. Then there is a infrared area between these two detector parts. When someone passes the infrared region, the detector will send alarm signal to the security alarm host (which not included here). Install Distance (Max.) in the above chart means the maximum install distance between the two detector parts. Great to be installed for security system in school, office building, hospital, home yard, etc. Detecting range can be reduced to 30% in bad weather conditions, like rain, dust, etc. English Usage Manual included in the pack. It will give you detailed instructions.