Plate armor covers the whole body of the adventurer. A character in my game has resistance to bludgeoning damage but not slashing damage. He was arguing that since the armor covered his whole body, he would still receive damage due to the fact that the attack hit, but since it didn't slice through his armor, it would have a large area smash into his chest, and therefore it would be bludgeoning damage rather than slashing. (Piercing could arguably go through the armor, because arrows and such.)
Since the attack didn't slash through his armor, and it was his armor that hit his skin, would it be bludgeoning damage or slashing damage?
Best Answer
As an addendum to Thomas Markov's excellent answer, I'd like to add the old adage that "HP are not meat points". 5 HPs of slashing damage does not directly translate to a slashing wound, and 5 HPs of bludgeoning damage does not directly break a bone. Rather, many tend to think of HP as a mixed pool of mental resolve, stamina, and vitality.
So while it makes sense narratively to treat slashing damage to plate armor as less effective, this does not result in a game-mechanical reduction of damage. The slashing swing with a longsword against your armor plate removes 5 HP from your "fighting resolve" pool, same as a bludgeoning hit from a mace would.