Notice the material on the smaller mannequin, the gray, silver color of the dress. A shiny material can have Refracted light which causes a Reflection in the valleys or deep parts of the folds. These folds are darker in tonal values on cotton for example. Drawing shiny material are small stokes, tedious lines of smooth value ranges. So, shiny material is classified as 102 level.
I added a few mannequins to my studio to help me and artist friends with Life drawing, foreshortening views and so on. I want you to notice how the eyeball is a sphere in a socket! Drawing spheres well will help you later on with drawing ans shading portraits.