It's all in the cards!
Henzell says...
  1. "Guaranteed damage reduction" conferred by {ac} that applies to melee attacks and ranged attacks from launchers (i.e. bow, crossbow, or sling) or thrown projectiles. Does not apply to attacks from spells, special abilities, or other items. Monsters do not have GDR, only players. If you're making equipment decisions based on this, please reconsider.
  2. GDR is [16 * AC^(1/4)]%. That includes base AC, enchantment, aux armour, you name it. When you roll 1dAC to subtract from attacks, if that roll is lower than GDR% of an attack's *max* damage, it's set to that % instead OR half your AC, whichever is less. There's a hard cap at 50%, which you won't hit.
  3. Why? To make combat a little bit less 'swingy' for high AC characters. Why only them? Can't answer that.

Information based on master branch and the great LearnDB.
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