Huh, it would be so awkward if I didn’t left a link to the source tutorial but can’t believe author have find out it so quickly
If all body parts are together, in the data table settings for the upper and lower arms are just used different meshes, like that:
http://genmindo.com/wp-content/uploads/2017/03/Gore_UpperParts-1.jpg
http://genmindo.com/wp-content/uploads/2017/03/Gore_LowerParts-1.jpg
http://genmindo.com/wp-content/uploads/2017/03/GoreMale_Parts-1.jpg