This rule is often presented as Preferred Enemy (X) where X identifies a specific type of foe. If the special rule does not specify a type of foe, then everyone is a Preferred Enemy of the unit. A unit that contains at least one model with this special rule re-rolls failed To Hit and To Wound rolls of 1 if attacking its Preferred Enemy. This applies both to shooting and close combat attacks.
The vehicle has the Preferred Enemy (Chaos Space Marines) special rule. In addition, it automatically passes Dangerous Terrain tests incurred for passing through ruins and similar terrain.