Concealment Bonuses to Saving Throws

Several Saving Throws result from powers that can be less effective if the target is hiding behind a wall, crate, or some other form of cover. System Shock and Poison will both work the same regardless of where the victim is, while Death Magic, Spiritual Powers, and Sleep/Charm effects target the person but do not need to physically hit them to have an effect - assuming the victim is visible, it does not matter how safe their hiding place may be.

Magical Weapons are generally rays, beams, or conjured creations that must hit a target, thus cover can give some bonus to avoiding them. Similarly, hiding behind a crate obviously gives protection against an Area Effect, like an explosion.

Percent coverage Saving Throw bonus granted to Save vs. Area Effect and Save vs. Magical Weapon
50% to 75% +1 bonus
75% to 95% +2 bonus

For coverage 95% and above, the target is essentially immune to any targeted attack or spell that is made from a distance further than melee range, unless the ranged attack is powerful enough to seriously damage or destroy the cover that the character is using as shelter, such as a large explosion or missile. Obviously, even if somebody is almost completely hidden, a person who notices them could still walk up to them and cast a spell on them in melee range, hit them with a sword, and so on.