1/4" Medium Heavy - used for more general sewing on most woven fabrics and common apparel applications like cotton pants.
1/4" Heavy - used for heavy apparel applications like corduroy pants, denim, canvas products.
1/4" Extra heavy - used for the heaviest industrial applications like tarps, tents, heavy canvas products, heavy awnings and very heavy denim.

Cutex Lap Seam Folder Part Number #23420Z-9 (1/4") for Union Special 35800 Class Sewing Machine