Critical Hit (X) "Attacks made with the Critical Hit (X) Special Rule have a chance to automatically cause a wound and inflict 1 extra point of Damage."
When any Hit Test is made for an attack with the Critical Hit (X) Special Rule, if the result of the Dice Roll, before any modifiers are applied, is equal to or greater than the value of X attached to this variant of the Critical Hit (X) Special Rule, then if a Hit is inflicted by that Hit Test, that Hit becomes a 'Critical Hit'.
A Critical Hit automatically inflicts a wound without any dice being rolled, counting as the roll of a '6' for any variable Special Rules that might be triggered by a Wound Test, and increases the Damage of the Hit by +1.
Phage (X) "Unsaved Wounds with the Phage (X) Special Rule reduce a Characteristic by 1."
If an attack made for a Weapon or Model with this Special Rule inflicts one or more Unsaved Wounds on a Target Unit, then once the Shooting attack or Initiative Step in which the Unsaved Wound was inflicted has been completely resolved, all remaining Models in the Unit that was the target of the attack must reduce the Characteristic that is the value of X for that variant of the Phage (X) Special Rule, but no individual Characteristic may be reduced by more than 1 by the Phage (X) Special Rule no matter how many Unsaved Wounds with any variant of that Special Rule are allocated to Models in that Unit.