The magic slate allows its user to store maps and view them at any time. It is given as a reward for completing the test of the slate, one of the three tests for becoming a wizard at the School of Magic.
After completing the test of the slate, at the Island of the Dome of the Slate, the rector of the school of magic appears and gives Twinsen the magic slate. With it he can store the map of the dome of the slate, a map of Citadel Island's sewers, a map of the Emerald Moon base, a map of Island CX, and a cross section map of Zeelich.
"By using the Action command in front of a plan, a print or any other illustration, you immediately create a copy with the Magic Slate. You can consult it at any time by selecting the Slate in the inventory."