The holding layer around a life — a creature's home, a command line's room, an empty row awaiting its filling.
Old English sciell / scell (a shell, a husk), from Proto-Germanic *skelja, related to shear (to cut, split); the holding layer around something living or something empty.
A boundary that can hold three different kinds of thing without changing shape — a life sheltered inside it, a command spoken through it, or nothing at all yet, waiting on what will fill it.