TFC introduces 4 damage types, general, slash, pierce, and crush. A weapon will specify what damage type it does, and armour will specify its defence rating against the three damage types. Some mobs are weaker or stronger against certain damage types. Swords and axes do slashing damage, maces do crushing damage, and knives, javelins and arrows do piercing damage. Mobs deal mostly either slashing or piercing damage.
General damage is unaffected by armour, and is sustained from falling, fire, suffocation and explosions. Endermen are the only mob to use general damage attacks, thus rendering your blue steel suit of armour completely useless.
Immune to piercing, but weak against crushing damage.
Zombies are strong against crushing, but weak against slashing damage.
Protects against 200 slash, 250 pierce, or 300 crush damage.
Protects against 500 slash, 500 pierce, or 330 crush damage.