A unit containing one or more Fear-causing models gains +1 to its Combat Resolution score. If the Unit Strength of all Fear-causing models is double or more, the Combat Resolution score is increased by +2. Note that the bonus from Fear is otherwise not cumulative for having multiple units with Fear involved in the same combat.
If the majority of the models in a unit cause Fear, they also gain Immunity (Fear), and thus the Combat Resolution bonus of their Fear-causing enemies is ignored for that unit.