Some weapons or creatures are so powerful that they can punch right through armour, so armour provides less protection against them. Such attacks confer modifiers chat are subtracted from the foe's armour saving throw. This is shown by the table below.
For example: A crossbow (S4) hits a warrior wearing light armour and shield. Normally he would need to roll 5 or 6 to make his save and avoid taking the wound but, because of the crossbow's enormous hitting power, -1 is subtracted from his dice roll. There, he must now roll a 6 to save.
Armour Save Reduction
3 or less