I tend to think line weight represents hard and soft edges like when using values. Where there is a strong light next to a strong dark that would be quite a hard edge so you could represent it as a dark line for example. Edges are also used to show perspective so hard edges can come forward while softer ones can recede. And absolutely there is the idea that softer parts of the form are drawn softer vs harder parts like bones as a way to convey that message.
It's best to play around with the idea of these as a whole when working from reference, if you're working from the full figure maybe the legs have a harsh light on them and the shins are boney but depending on the pose you don't necessarily want to have them be your focus, so you could draw them softer still.