GHC Generic Rep Type Parameter

In GHC.Generics, the representation types are all of kind Type -> Type (i.e. the kind of Rep is Type -> Type). If you look at their definitions below you can see that the type is unused (and probably from the name p which I presume stands for phantom), . data V1 p -- lifted version of Empty data U1 p = U1 -- lifted version of () data (:+:) f g p = L1 (f p) | R1 (g p) -- lifted version of Either data (:*:) f g p = (f p) :*: (g p) ...