This option controls the depth of the armhole in the Brian block Aaron is based on.
This option applies to the new v3 way of calculating the armhole depth. If you enable the legacy armhole depth option, this option will not apply, but instead it’s legacy version armhole depth factor will.
In any case, Aaron allows the armhole to drop much further using the armhole drop option. So you probably don’t need this option from the underlying block, but it’s there if you’d like to tweak it.