The Mass Storage system allows players to virtually store stackable items, which can be accessed at any time, from any survival world, via command.
As stated in the opening paragraph, the Mass Storage system is intended to store stackable items that would otherwise cause clutter in your chests. As such, most non-stackable items, as well as specialty items (enchanted tools and armor, written books, and potions) cannot be stored using Mass Storage. The only exceptions are saddles, boats, music discs, and horse armor.