Garrett AT Pro Owner's Manual page 25

The examples shown below illustrate how an iron object
can often "mask" out the signal of an adjacent good
target when too much iron discrimination has been applied.
Using the AT Pro's High-Res Iron Discrimination, apply just
enough iron discrimination to reject the undesired iron nail
shown in this example. By using only a minimal amount of
iron discrimination, the detector will detect the combined
conductivity of the coin and nail together thus overcoming the
potential "masking" problem.
Illustration 1 Illustration 2
Iron targets, such as the nail shown in Illustration 1, can sometimes
mask a good target's signal. If too much iron discrimination is
applied, the good target (seen in Illustration 2) can be missed.
Read page 26 to learn how to apply the proper amount of iron
discrimination to eliminate the nail shown in Illustration 1 and still
detect the good target shown in Illustration 2.
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AT Pro International™