From Dark and Darker Wiki
Adding on to Discussion, reading the description of the perk it says it reduces True Damage by -3. Sounds to me that it reduces every incoming attack type by -3.
Example: A Build consistent 30 Physical Weapon Damage (PWD), 9 True Physical Damage (TPD) and 3 Additional Physical Damage (APD)
If I'm correct, it will reduce each thing individually 30 PWD - 3 | 9 TPD - 3 | 3 APD - 3
I'm not sure if APD is also included, would appreciate if someone can confirm this!