This phenomenon was discovered by Levi the first time he entered the Ashen World—only storage spaces for dead objects can be opened. (See Chapter 215.)
Unless transcendent creature spaces are built upon other planes, they cannot be opened in other planes.
So far, according to the wizards' research, there are only two types of living creature spaces that can be opened without any plane restriction.
The first type is something like the School of Spiritualism's "Spiritual Space"—this space is quite unique, as it is created within the body itself, essentially making the space a part of the body.
The second type is a Legendary Wizard's "Legendary Demiplane"; needless to say, it is essentially a higher-level internal space.
Watching the transcendent creatures gradually disappear into the mountains and forests, Levi turned and left.
...
Holy Brilliance Calendar, Year 1130, Month of Flowers.
