The ATN scope has you put a line across the top and bottom of an object and based upon the size of the object will estimate a distance.
If u want a more accurate distance for the ATN, get the ABL (2nd picture) that will attach to the front of the scope and will need to be paired via Bluetooth to the scope.
if you use the 1st picture or any regulat rangefinder like a halo you can simple take a reading from the rangefinder and manually dial it into the scope.