imagine player A attacked with a bloodthirster and an ironclaws horde and blayer B defended with 2 ironbreakers of ankhor with x=3 (-> toughness 3).
the bloodthirster would deal 5 uncancellable damage (right? the text says "damage cannot be cancelled."), while the horde would deal 4 damage.
the first 3 uncancellable damage could kill one ironbreakers (because they have 3 hp), right?
and the remaining 2 uncancellable damage would be assigned and applied to the second ironbreakers (since it circumvents toughness), but A would have to use all 4 remaining cancellable damage to kill it (because it has toughness=3 only 1 damage would come through, which would then suffice to kill it), right?
now imagine an effect that gave A's bloodthirster +1 axe, would this 6th axe also be uncancellable, i.e. would A be able to kill both ironbreakers with his bloodthirster's damage and the 4 damage from the horde come through?
sometimes i wish the damage tokens had two different sides, so that one could represent assigned and applied damage differently.