If a unit is made up of creatures with multiple Wounds you have to remove whole models as casualties where possible. This is a convenient method which saves on both brain cells and book-keeping. For example, if you had a unit of Ork Nobz (hulking great things with 2 Wounds each) which suffered 5 Wounds from bolter fire, you would remove two models as casualties and note down or place a dice by the unit to indicate that 1 Wound was carried over. If the unit later took another Wound you would have to remove another Ork Nob.
If the hits are powerful enough to kill outright take off one model as a casualty for each wounding hit inflicted. For example, Ork Nobz are Toughness 4, so a lascannon (Strength 9) can kill one Ork Nob with each hit. Hence two lascannon hits on a unit of Ork Nobz would remove two Nobz as casualties. Note that these 'instant kills' do not affect Wounds that are being carried over, these are retained so that they can be made up to full kills by other less powerful hits.