Dual's function has been known for a long time; it's basically an anti-regen that decreases your HP instead of raising it. It's actually two-statuses with an invisible/unnamed one (called Dual-Drain or Seizure) doing the actual work while Dual itself does nothing except appear in the status ailment box when using certain items. But DD/Seizure can't work if Dual isn't inflicted, unless it's in the Bottomswell fight.
Hidden element is a functioning element that was (almost) removed from the original game. To an extent, the game has a limitation where Materia needs to have an element (it can't be non-elemental) so Hidden was used for that. Problem was, a lot of enemy attacks in the JORG version carried this element and some attacks still carry it in the later US/EU releases like Ultima Beam so you could use a 'non-elemental' Materia with elemental to block those attacks.
For NT, I decided to use Hidden as a counter to Holy; every enemy that looks demonic or has some sort of association with Jenova (SOLDIER enemies, Hojo's experiments, etc.) will be resistant to Hidden but weak to Holy. Hidden is effective against all the Weapon bosses like Diamond, etc as well. That means giving the player some Hidden-element attacks for their arsenal, and Shadow Flare/Pandora's Box fit the bill quite well. You can also deal Hidden element damage by combining any non-elemental Materia with Elemental in your weapon.
