In the new FAQ the attacker has to assign damage enough to destroy all defending units, before any damage can be assigned to defending capital. So it's Unit HP + any active damage cancellation effects at the defending units, when you go to Assign damage phase. With Toughness this is pretty simple as it's Current HP + Toughness value = amount of damage to destroy unit, but how this rule is applied when the defending unit has infinite damage cancellation?
A few example units:
#46 High Elf Unit - Dragonmage - 5R/3L - 2P, 3HP - Mage. Elite. "Whenever this unit is assigned damage, cancel all but 1 of that damage."
#23, 1x Unique: Gustav the Bear (Empire Unit) 4-EEE 2/3 Hero. Cavalry. Cancel all damage to this unit while any opponent controls a corrupted unit.
No amount of damage can be assigned to these units in order to destroy them and both of those abilities are present when you go to Assign damage phase. So does the attacker have to assign all of his damage to these units, before any can be assigned to the capital or is 3 damage still enough? I'd say it's the former if you follow the FAQ'd rule, making these units pretty good defenders.

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