It's based on how Tenkaichi split up different versions of characters like Goku and Vegeta on the roster. Mid refers to the version that is between the start of the Namek Saga and the end of the Androids Saga. Goku (Mid) had a unique Super Saiyan hair, looking much more like flowing yellow and orange flames rather than solid, individual yellow spikes like every other character had which is what the hair on this mod is supposed to look like, but it not having physics makes it just look like an ugly mass of chewed gum rather than actual hair.
