The unassigned skin followed by an assigned skin is for normal mapping, e.g.
Skin1 -> assigned to subset 1
Skin2 -> unassigned (normal map for subset 1)
Skin3 -> subset 2
Skin4 -> unassigned (normal map for subset 2)
Skin5 -> subset 3
Skin6 -> unassigned (normal map for subset 3)
Skin7 -> unassigned
Skin8 -> unassigned