The Veil is the common term for the membrane between the Universe and pocket universes. The term is also applied to the space between successive pocket universes which, if they exist, are accessed linearly. Objects in the veil effectively exist in two realities until transitioned to one or the other. Objects in the veil are not visible to either reality, though both realities are visible from the veil. Whichever reality is “closer” is the more sharply defined of the two.
The Veil exists only inside the gravity wells of a small number of planets – most habitable, some not. Each Veil functions as a single planet- or moon-wide access point to a unique pocket universe. The ability to move between realities is a learned skill involving a combination of genetic predisposition and training, and while not directly related to the ability to transition objects in and out of Am, the mental process is quite similar.
Due to the absolutely massive security issue the Veil poses, the vast majority of human military and otherwise secure installations have been constructed either in space or on worlds that do not have pocket universes attached to them. Military security from Veil-based threats was one of the principal factors that moved stellar humanity into space five centuries before the humans of Earth launched their first rocket.