1. Are they passable from the interior? That is, can you still shoot physical projectiles from inside?
2. Can one punch a hole in only part of a sigma? This is what Aiken Drum apparently does to Pallol One-Eye in their duel.
3. Can they be made transparent? Opaque? Trnaslucent? My impression was that they looked like mirrors from the outside but it seemed that Nodonn could see Aiken inside his shield.
4. What happens if a structure (such as a building) is covered by a sigma too small in diameter to contain it? Does the field phase right through the walls at its perimeter? Does this cause any structural damage?
5. Are they required to be spherical? Of those that are spherical, are they hemispherical or full spheres? Of those that are just hemispheres, do they cut off so that they are "open" on the bottom, or is the "flat side" closed off?