This Elite Security guard is armored.

The corps are now issuing high-tension personal armor to their security forces as standard equipment. I think they're getting scared of us. It would be flattering if it didn't represent an existential threat. Make sure you bring along something extra pointy for emergencies.
—Central

Many guards have armor, which prevents some attacks against them.

All attacks that can deal KO or lethal damage have an armor piercing value. If a guard has more armor than an attack's armor piercing value, then the attack is prevented against the guard.

A prevented attack simply never happens:

• Buttons to perform attacks with insufficient armor piercing against guards are disabled. If these buttons are hovered over, “ARMORED” is shown as the reason why they cannot be performed.
• Ambushing units readying attacks will not attack guards with more armor than their attack is able to pierce that walk next to them.
• Overwatching units will not attack guards with more armor than their attack is able to pierce when they act in their sights. Note that a turret is able to overwatch a guard with more armor than it is able to pierce, causing that guard to stop moving for the turn; it just won't be able to shoot when that guard moves again.

Each guard type has an inherent armor value. This value can be increased by the Chiton daemon, and reduced by the Aces algorithm.