Flat-felled seam allowace
Controls the amount of seam allowance on flat-felled seams.
The armhole and side seams on the shirt are flat-felled seams, and those require extra seam allowance. How much is a matter of personal preference, since some people prefer wider flat-felled seams, while others have a preference for more narrow flat-felled seams.
This option controls the amount of flat-felled seam allowance as a factor of the regular seam allowance.