Smart Keys are keys such as Infernal Compasses and Nightmare Sigils required to enter the Infernal Hordes and Nightmare Dungeons. Their use is universal, across all difficulties so for example if you activate an Infernal Compass on Torment 2 you will enter a Torment 2 difficulty Infernal Horde, if you activate it whilst on Pentitent difficulty that will be the difficulty of the Infernal Horde.