Quick Question About Armor Sets

I've recently acquired the Twilight Leather Assassin set, but I'm missing the pants and cannot find them on the Auction House or player shops.



On the normal light armor - not assassin type - it says, "Bestows an Assassin or enchant type set option", can someone please explain what this means and how I would go about doing it? I assume the normal light armor will not complete my set, as it would need to be assassin type.



Thanks,

Dean.