The term inscribable applies to both armor and weapons. Inscribable armor means you are able to apply insignias, a prefix upgrade to your armor. Inscribable weapons allow for inscriptions to be applied. Inscribable weapons can only be found in Nightfall and Eye of the North, as well as in the end chests in the elite areas The Deep, Urgoz's Warren, The Fissure of Woe, and The Underworld. To apply an inscription or insignia, simply double click on the inscription or insignia and then double click on the inscribable item. You can always upgrade, but the latter one will be lost and you can also salvage the inscription or insignia, but salvaging an item runs the risk of it being destroyed in the process.