Hardness isn't secret; generally speaking, people will just not take attacks that hardness nullifies. So I envision hardness as "Some armor is so effective that it's especially hard to find an opening."
Originally posted by Ren9077
The characters shouldn't have full knowledge of the combat system, but that combat system should be trying to model what's happening in-character. Knowing that really impressive shiny armor can no-sell attacks isn't exactly a secret that one can only discover via trial-and-error. When my PC is faced with a heavily armored enemy, he knows in-character that he's going to have to hit them better to make them feel it. While my character is not aware of Initiative as a discrete quantity, he can spot openings that he has created and recognize an opportunity to strike a telling blow. I, as a player, am seeing the numbers and making math assessments that my character isn't aware of, but those numbers represent things that my character is very aware of.
Yes, this. Player knowledge doesn't map one to one to character knowledge, but players making choices based on player knowledge models characters making choices on character knowledge.