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Exceptional items are the name given to items with special mods that can not be recreated through traditional means, such as using weapon upgrades or insignias.
Guild Wars does not specifically name these items and therefore to help clarify the difference the user-created term of "exceptional" was made to make a distinction against "unique" items.
"Exceptional" items might be quest related, pre-nerf, or unique in nature. By definition you can not find exceptional items normally through non-unique drops.