ZAJ has said any of this is subject to change and I will update this accordingly.
Your PEN & RES can be found with -stats
Most new weapons & armor have the new stats, especially when you level them past 30.
The way it calculates is any incoming DMG gets reduced by AC, then DR and then finally RES in that order with any leftover DMG hitting directly into your HP, not including any buffs or items that add other effects.
RES works almost exactly like DR, so if you have 100 RES it would be the same as having 300 DR, except it has the added bonus of blocking PEN damage.
PEN adds damage to magic and normal attacks. If you have 100 PEN you would do 100 more dmg per attack, whether thats with a spell, arrow or melee. It also ignores all DR and is only resisted by RES.
- 1 DR = 1 BLOCKED DMG
- 1 RES = 3 DR
- 1 PEN = 1 DEALT DMG
- 1 PEN = 1 RES
- PEN IGNORES DR & AC
- DMG hits AC then DR then RES last
So if you get hit with 300 DMG & 100 PEN and you have 100 RES the RES would absorb the normal dmg, but be unable to block the PEN DMG as it would be all used up.
If you had 200 RES it would absorb all the dmg.