Let's make an example, so you see what I am talking about. Since this actually happened, the example is maybe a slight bit more complex than needed:
Let's assume the imperial player wants to attack the Space Port Primus, where a traitor horde is assembled. The Adeptus Mechanicus, including the Fabricator General, has fortified the area west of the Volcanus Factory Complex and is holding out with a Titan and two tank regiments, and the traitors have breached the border between that area and their space port. Jaghatai Khan and two regiments of White Scars Space Marines had to retreat from the Fortress of Truth in an earlier battle, but have regrouped and join to support the Assault order from a non-fortified area to the north.
So the situation is: Assault order, two origin areas, one target area. One origin area is fortified with a breached border, the other origin and the target area are not fortified.
So now, if the defender wants to assign damage to the attacker: How is this handled, how much damage does he have to subtract from the regular damage he generates, due to the fortification he has to overcome in only one origin area? The rulebook does not explicitly state this, I think, or I have overlooked it...
The way I think handling this is reasonable is as follows: If the defender wants to apply damage to units outside the fortification, he does not have to reduce his damage. If however he wants to switch to assigning damage to units in the enemy fortification, he has to reduce the remaining damage before he can apply it.
Is this correct, or is there some special rule akin to the one concerning crevasses that I have overlooked?