Just figured I'd point out some errors in what is noted here. Neither of the people quoted seem to have taken the time to look up the actual formula used for the game, which are available on the
official wiki.
Damage reduction is countered by armor pen. The relevant formula being the damage dealt formula which is:
Damage Dealt = Damage * [1 - EffectiveArmor - DamageReduction + Penetration] * (1 + CriticalMultiplier)
Note that the number in the [] cannot exceed 1, convert percents to decimals for variable. Also you only add the crit multiplier on a critical hit.
As can be seen by this formula, Penetration directly counters Damage reduction on a 1 to 1 basis. Effective armour I will cover below.
As for the bit about evasion being countered by +100 accuracy skills, this is generally accurate, as hit chance is determined by accuracy - evasion, and rarely will a tank won't come close to exceeding 100 evasion drop that result below 100 after the 100 is added unless said tank is using a skill that pumps evasion similarly high, but much the same could be said of damage reduction tending to be countered by skills that massively pump pen (of which there are a few, though admittedly not so many as there are ones that pump accuracy).
This comment is wrong, in that armour is a percentile reduction. The relevant formula here is the effective armor formula:
Effective Armor = 100 - (25900/((Armor/1.01) + 260))
Note that based on this formula, 0 armor provides about 0.4% effective armor, while 100 armour provides a little less than 28% effective armor. As can be seen in the formula for damage above, effective armour is the same as damage reduction in how it impacts damage, and is directly countered by penetration.