Some models have more than one Wound on their profile. Should a unit of such models lose Wounds, you must remove as many whole models as possible. In other words, Wounds are lost by a single model until its Wounds characteristic is reduced to zero. Wounds are then lost by the next model, and so on. You cannot spread the loss of Wounds throughout the unit to avoid casualties.
For example, a unit of Ogres, each with three Wounds on its profile (W3), loses five Wounds. One Ogre will lose all three of its Wounds and be removed as a casualty, and one Ogre will lose two Wounds, leaving it with only one Wound remaining.