Sometimes a unit will contain creatures with different Toughness characteristics. When this is the case allocate hits before rolling to wound. You may only allocate one hit against each model until all of the models in the unit have taken one hit each.
Diagram: Allocating Hits — the Space Marines shoot and score four hits against a mixed Ork unit (Orks, an Ork Nob, and Gretchin, which have different Toughness values). The Ork player allocates three hits to the Gretchin (T2) and one hit to an Ork (T4) before his opponent rolls to wound, illustrating that hits must be spread one per model across the unit before any model can take a second hit.