This key is my invisiblity key and contains a different invisibility depending on which class you're using it on. Wizards and Mages both use Group Perfected Invisibility and Clerics use Group Perfected Invisibility to Undead. You can get both of these Invis abilities to stack on your characters too if used properly! For me I have to be on my Clerics screen that way the Cleric casts his invis first and it lands just moments before the Mage/wizards invis.
Usually this means at least one person in your group will end up without one of the two invis' cast on them... However it'll give the rest of your group the means of navigating through a tough area with undead and regular enemies. Worse comes to worse if you can't get invis to land on all 6 of your characters only 3 of them need to make it to the location you want to camp; you can just drop a campfire to bring anyone to you that didn't make it.
Note: If you're using a class with an AA invis that isn't listed above I highly recommend you do not use their invis or even bother placing it on your bars as the same key. If you're a Shadow Knight for example and you place Cloak of Shadows on your bar - you'll end up breaking the invis your boxes cast because you'll still be casting the spell when their invis hits you.
This won't be that big of a deal unless you are trying to stack regular invis with IVU. Or if you're a class like Beastlord's and you have a really terrible invis that still has a time limit/you have to target someone to cast. Assuming you're always going to be boxing it's better to just let one character do the invising (or two if you have IVU too).