Each one contain landscape that mirrors the one in the real world, but with no living things present, and with damage done to it not affecting the real world. Their size was originally a few kilometers in radius, but as more cards were collected, they started shrinking until the last one was the size of single building. Their name comes from the analogy that if you put two mirror opposing each other, they would reflect each other infinite number of times, and each pocket space is but a single of such reflections, a mirror image of the world.
Each Class Card was believed to be contained in a separate space, although it was shown that two cards could share a single space.
Mirror World could only be accessed by Kaleidosticks with a formed contract by opening a “Mirror Road” via a spell. After its formation a Mirror Road could immediately transfer several humans to and from the Mirror World before closing.