The target unit suffers casualties for any wounds that have not been saved. Casualties are always removed from the rear of a unit.
For every model that fails its save the target unit suffers an unsaved wound (any saves that were not possible because the modifier was too high also count as having been failed). Most models have only a single Wound on their profile, in which case a single model is removed for each unsaved wound caused. Individual warriors are not necessarily dead, they may just be too badly wounded to fight on. For our purposes, the result is the same, so we treat all casualties as if they were killed and remove them from play.
Although casualties would normally fall amongst the front rank, for the purposes of game play we remove models from the rear rank of the unit. This keeps the formation neat and represents rear rankers stepping forward into gaps formerly filled by fallen comrades. If the unit is deployed in a single rank, then casualties are removed equally from both ends.