Armor penetration works how you think it works: 12% penetration reduces the target's resistance by 12% during damage calculations; 50% reduces it by 50%; etc.
The hard cap changes each level, but I noticed the armor buff from Phase in the Champion System grants about 1% mitigation in armor. At least at VR14. At VR14 the hard cap for Resistances is 32,500, every 650 equalling 1% mitigation. Phase grants 640 Armor. Any Armor over the hard cap is not null, as it acts as a buffer against armor penetration.
Armor penetration works how you think it works: 12% penetration reduces the target's resistance by 12% during damage calculations; 50% reduces it by 50%; etc.
The hard cap changes each level, but I noticed the armor buff from Phase in the Champion System grants about 1% mitigation in armor. At least at VR14. At VR14 the hard cap for Resistances is 32,500, every 650 equalling 1% mitigation. Phase grants 640 Armor. Any Armor over the hard cap is not null, as it acts as a buffer against armor penetration.
@Ffastyl , the "hard cap" for both Spell and Physical resistance is 32.5k? If you were to go over in resistance, which would be better to do so? Spell or Physical? I was toying around with a build last night that when I popped Unstoppable, I was at over 50k Spell and 22k Physical resistance. My build prior to that was around 40k Spell and 13k Physical and I was holding up against a really good NB for a while but couldn't take them out as I was getting hit pretty hard, they were duel wield. I want to go for greater survivability in PVP and not waste any resources.

Armor penetration works how you think it works: 12% penetration reduces the target's resistance by 12% during damage calculations; 50% reduces it by 50%; etc.
The hard cap changes each level, but I noticed the armor buff from Phase in the Champion System grants about 1% mitigation in armor. At least at VR14. At VR14 the hard cap for Resistances is 32,500, every 650 equalling 1% mitigation. Phase grants 640 Armor. Any Armor over the hard cap is not null, as it acts as a buffer against armor penetration.
@Ffastyl , the "hard cap" for both Spell and Physical resistance is 32.5k? If you were to go over in resistance, which would be better to do so? Spell or Physical? I was toying around with a build last night that when I popped Unstoppable, I was at over 50k Spell and 22k Physical resistance. My build prior to that was around 40k Spell and 13k Physical and I was holding up against a really good NB for a while but couldn't take them out as I was getting hit pretty hard, they were duel wield. I want to go for greater survivability in PVP and not waste any resources.
Thanks to some broken mechanics which I will not mention here, Armor Penetration of 38.8k+ can be easily achieved. It has to do with a certain weapon with a certain trait with a certain champion point skill that required only 1 point spent in it.
Might as well be wearing toilet paper armor
Both have penetration stats, so it is a debate over which is better to "overcharge." Generally people say Spell Resistance because it is easier with Nirnhoned as it is. For Physical Resistance, 20k is the benchmark for "good" defense. Higher is nice, but up to you.