Talk:Damage 2.0/Viral Damage/@comment-98.94.18.40-20140121152158/@comment-63.230.18.117-20140310031720

https://forums.warframe.com/index.php?/topic/134007-warframe-powers-post-u12/?view=findpost&p=1762321

at least as of U11.3.3, modifiers against armor types are accounted BOTH as a multiplier for the base damage AND a mitigation factor for the armor value itself, though only for the portion of a hit's damage that is that type. So the corrosive/radiation/puncture portion of damage will deal multiplied damage to applicable armor types, AS WELL as being applied to a reduced armor value, but not the other components of the damage.

Unfortunately, not a lot of people seem to be doing much in testing (or datamining) on this info, so it's hard to say how accurate this still is, but afaik right now that thread is the best source we have on how damage type multipliers apply to armor.