it's a better version of point track. the box expands until it finds a "point trackable" target then the box closes around the target with the camera autocentering on the box.
Point track however only has the 2 bars. once a target is found there is little difference between the two, but auto track has an easier time finding the target than point track which only checks the spot between the bars for a short time.
probably would be more useful in a/a situations when trying to acquire an aircraft that your radar does not have. Think of it like using the EO on a flanker but simpler.
