Garrett AT Pro Owner's Manual page 26

Example: Detecting Targets in Trash with High-Res Iron Discrimination
In the illustration above, the AT Pro is operating with an IRON DISC setting of
20. The nail seen in Illustration 1 (on page 25) registers from 10 to 25 on the
Digital Target ID scale. To eliminate the nail from detection, increase the iron
discrimination level to 25 using the IRON DISC (+) pushbutton.
In Illustration 2, the same iron nail is laying above a good coin target. Since
the Iron discrimination level is set to 25, the nail by itself would not be
detected; however, the two objects have a combined conductivity of more
than 25.
Therefore, the good target is detected due to the combined conductivity
being higher than that of the discriminated target (nail) alone.
26 Garrett Metal Detectors