Focal length is a parameter of the camera, it’s the length in mm between the lens and the sensor. Focal_dist is a parameter relative to the distance between the subject and the lens, and is calculated as explained earlier in this FAQ. Focal distance is relative to focus, it can only be used with fixed focus (no autofocus). 

Focal length of a camera determines the viewing distance and the viewing angle of a camera. Focal_dist is not used to determine the maximum viewing distance, but to calibrate the camera for distance calculation.