This option allows you to use the legacy way of calculating the armhole depth in Brian, which is the block Aaron is based on.

The legacy (v2) way used the biceps circumference to estimate the armhole depth.

The v3 way instead uses the waist to armpit measurement to locate the bottom of the armhole.

If you enable this option, the Brian block will revert to the v2 way of calculating the armhole depth, which will influence the armhole on Aaron too.