From Diablo Wiki
Like other forms of damage, Physical can be mitigated by physical resistance. It it also affected by armor.
Diablo 3 classic has a bug with physical damage that is most commonly seen on the so-called "black weapons." This causes some +%damage modifiers to deal much higher damage when modifying "black" weapons than when acting upon all elemental types of damage. See the black weapons article for full details.
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