A list which lets us define who can not interact with an element.
With only a permission list no combination of sensors can distinguish between:
Only an authorized person is in range
Both an authorized and any other person are in range
For example, you can not have a door which automatically opens only when you're alone.
It would also help attempts at starting civilizations, since it would allow having public spaces by default and banning particular people from interacting or building in them.