the term "shell" is just an established term for an anti-magic protective bubble, very prevalent in the final fantasy series, i am fairly certain i have seen it in other places as well.
as oldtrees and mav stated, it does not function against abilities/skills/effects- just spell cards, and it does not have a reflective element, that clearly distinguishes shall from reflective shields, but shell also helps protect your creatures.
my barrier card, currently in level 1
, i believe, is in a sense the other half of this card's theme, which helps protect your creatures from your opponent's creatures.