It’s like the defense mods. A defense mod increases both armor and resistance. Defense penetration goes through both as well. Armor only affects physical offense, and resistance only special offense. I don’t have any characters with greater than 0% defense penetration, so I’m guessing that’s not something we have gear for yet. Some characters gain defense penetration in certain circumstances or with uniques, like Boba, and that has a percent impact on both armor and resistance penetration. It is definitely confusing. In fact, swgoh.gg doesn’t even display the defense penetration stat yet.