STARTING STRENGTH, HALF-STRENGTH AND DESTROYED UNITS
The number of models a unit has when it is added to your army is known as its Starting Strength. Throughout a battle, models will suffer damage and be destroyed. When a model is destroyed, it is removed from play. While the number of models in a unit is less than half its Starting Strength, that unit is said to be below Half-strength. When every model in a unit has been destroyed, the unit is said to have been destroyed.
If a rule is used to split a unit into multiple units during the battle, the Starting Strength of each individual unit is changed to be equal to the number of models in that unit. If several units merge together to form a single combined unit during the battle, add the Starting Strengths of all the individual units together to determine the new Starting Strength of the combined unit, and use this value to determine if the combined unit is below Half-strength.
Some rules will only trigger if an enemy unit was destroyed by you, or by a model or unit from your army – this means that the last model in the enemy unit was destroyed by an attack made by a model in your army, or it was destroyed because it fled the battlefield, or it was destroyed by a mortal wound inflicted by a rule that a model in your army is using, or it was destroyed as the result of any other rule that a model in your army is using that explicitly states that the enemy model is outright destroyed. Enemy units that are destroyed by any other means are not destroyed by you, or by a unit or model from your army.