p. 44 states: Miniatures that are not obscured must be assigned damage before miniatures that are obscured.
Imagine the following situation: you deal several points of damage to a unit. That unit includes a hero. The hero is not obscured, while the rest of the unit is in cover, but visible.
How do you divide the damage?
a) The hero gets all the damage until he is dead or there is no more damage left. Surplus damage is be divided among the rest of the unit.
b) The hero gets the first point of damage, the rest may be divided as seen fit.