Armor
Certain xenomorphs have armor, which reduces incoming damage by a percentage each time they take a hit. Specific castes have armor which aids in their survivability, notably the crusher and defender. Armor itself is linked in with health, reducing damage. It is similar to health in that it will regenerate overtime. Yet armor generates slower. The difference between health and armor is that, you can still die even with a lot of armor remaining, it only reduces the damage taken.
Armor hardness is how much armor there is on a xenomorph. The crusher, having the most armor, will be able to sustain a lot more damage than a xenomorph with lighter armor.
The tiers in xenomorph armor is as follows: Crushers Defenders Queen, Burrowers, Praetorians, Ravagers Spitters, Warriors, Hivelords, Carriers
When taking damage with armor, it factors in the damage's armor piercing ability and the xenomorph's current armor status. For example, if a crusher were to be shot at with a rifle bullet with full armor, the crusher would take a very small amount of damage, due to having full armor. The crusher will still take damage but the damage is heavily reduced and the armor will also take a hit, decreasing it's effectiveness.
But, if an armor piercing rifle bullet were to hit the crusher, they would take full damage from the bullet as well as their armor taking a hit. It will reduce the armor's effectiveness the next time they take damage from projectiles.
When your armor has taken damage, there is a 2 second grace period before your armor is able to be damaged again. It occurs when 25% of your armor has been destroyed. This does not mean that you won't be able to take damage, it means your armor won't take any more damage for the duration of the grace period.
Do remember Armor =\= more Health . It reduces the damage taken to your health.
Evasion
Xenomorphs that don't have armor have an added bonus to projectiles missing them. Lurkers, drones, runners and larva have this bonus. The xenomorphs evasion will increase as they mature.