
Magic Resistance cannot be reduced in any way, meaning a Magic Immunity cannot normally be broken. While Characters have a cap of 50% Physical Resistance, monsters can have >99%. Physical Resistance is actually referred to as Damage Resist for monsters and Damage Reduction for Characters. The two "Resistances" that have a lot of caveats are Physical and Magic. While there are a few exceptions to the rule, once you're in Hell difficulty, every monster you encounter is Immune to at least one, and potentially multiple Damage types because they have >99% Resistance. Pit Lords, for example, are well known for their devastating inferno and their Fire Resistance increases from 70% in Normal, to 120% in Hell. Most monsters in hell have >99% Resistance of their "primary" Resistance, and this aligns with the damage type they do. Similar monster types tend to have the same "primary" Resistance, which increases in Nightmare and Hell difficulty.

Monsters with >99% Resistance are completely Immune to that damage source.

Each 1% of a Resistance decreases the damage received from that source by 1%. Fire Resistance decreases Fire Damage, Cold Resistance decreases Cold Damage and so on. Resistances in Diablo 2 provide a reduction in damage taken from a source of the damage type.
