Sometimes a unit will only be partially in cover (ie some of the models are in or behind cover and some of them are out in the open). In this situation the owning player has to remove casualties from amongst the models in the open first until there are none left – they are the more obvious targets and will suffer commensurately.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): When shooting at units partially in cover, hits are allocated to models in the open first. After hits are allocated, saves are taken.