A Hollow is a type of pocket that functions as a fold in space that can only be created by a wizard. Any number of objects can be put in a hollow and it will take up no discernible space, at least not in their dimension. There will be no bulges in the pocket, nor will they weigh anything. A limit to the amount of things a hollow can hold has yet to be discovered. However, the size of the opening limits what can be placed inside. If the object is too large to place within the opening to the "other plane" it cannot be stored.
Living beings can survive being inside a hollow. Kale's minor dragons used smaller "pocket-dens" inside her cape to rest and Toopka has traveled multiple times inside one of her cape's hollows. She described the experience as being uncomfortable as the other things in the hollow would sometimes bump into her and it was quite dim inside. She was capable of breathing, but sound was muffled so she would often poke her head out of the opening to tell what was happening.