get
Damage reduction this piece contributes under the legacy (1.8) armor model: its own armor points on top of the rest of the kit, plus its own protection enchantments.
Armor points give a flat 4% each (capped at 80%) and only Enchantments.PROTECTION - plus Enchantments.PROJECTILE_PROTECTION while considerProjectileProtection is enabled - reduce the damage further. Fire/Blast Protection and any non-defensive enchantments are ignored on purpose so they can never beat actual armor points.
1.8.9: net.minecraft.entity.EntityLivingBase#applyArmorCalculations 1.8.9: net.minecraft.entity.EntityLivingBase#applyPotionDamageCalculations