Can Zarathur use his Interrupt on every instance of damage assignment with Area Effect (because the text is "an enemy unit"), or is the simultaneous damage assignment to all units the same trigger, so only one can be increased?

Relevant text on Z:

"Interrupt: When damage is assigned to an enemy unit at this planet, increase that damage by 1."

~Andrew

Yes it can be used everytime.

Another way to think of this is that each unit will go through the "deal damage" process separately; i.e., there is an individual "assign-shield-take" step for each unit.

Ultimately, Zarathur can interrupt and add 1 to the damage assigned to each unit hit by an area effect for the same reason you can use a separate shield card for each unit hit by an area effect.

Hmm...

Page 6, "Dealing Damage":

"If a single source attempts to damage multiple units, all damage is assigned simultaneously. [...] All damage is then placed on the units simultaneously."

Does the above not mean there's only 1 trigger to respond to - the simultaneous assignment of Area Effect Damage?

~Andrew

No. It responds to damage being assigned to a unit. Each unit is assigned damage and therefore each unit is eligible for the interrupt. Even though it's simultaneous you can think of the interrupt to each being simultaneous too. It's multiple simultaneous triggers.

Trust me this came up in playtesting too and was answered by Nate.

What it comes down to is that if 3 units are damaged by the same effect, even though you "assign-shield-take" for all 3 simultaneously, each unit provides an individual interrupt/reaction opportunity. It's no different than in other games, when multiple characters are killed by the same effect, each one provides individual interrupt/save opportunities.