does this perk give resistrance to all forms of dmg such as energy dmg and straight dmg? or just reguler dmg

Ballistic damage. Bullets and melee weapons. You've got Refractor in Perception for Energy resistance, and Rad Resistance in Endurance for, well, radiation.

No... it gives resistance to physical damage and bullets... energy damage is completely different and is increased with perception thru the refractor or something perk which gives energy resistance.

...but there's no point in taking this perk with a power armor, right?

i mean +50 dmg resist aren't noticeable if i have almost 1.9k inside my X-01?

No point in power armor what-so-ever. An extra 50 is neither here nor there. It's practically another suit of armor if you're the type to play without power armor. Really, this game is designed such that difficulty is almost entirely in your hands, in a "you don't have to use it" way.

How does DR even work in this game, anyways?

Looking it up, http://fallout.wikia.com/wiki/Damage_Resistance#Fallout_4. It's not a flat percentage reduction like in older games, it's a percentage reduction based on the damage of the attack going in. So higher DR provides diminishing returns for protecting against low DR attacks, but are substantially more helpful against extremely powerful attacks.

Damn. This forum argued bitterly about Fallout 3's DR system vs New Vegas's DT system, and how both were really crappy in different ways... and this is completely different from both, and it doesn't seem to make any weapon or armor types useless.

Morrowind had a damage reduction formula that was based on the ratio between damage and armor and I always thought it was the best armor system Bethesda have done.

I'd love to be sure about the actual numbers, but I love the idea here.

I'm also a fan of the linear reduction provided by damage threshold from New Vegas, mostly because it's easier to conceptualize. But Bethesda don't seem to enjoy that kind of thing.

I thought Morrowind's system was similar to DT in New Vegas, where it just subtracted damage. And the problem with New Vegas is that you could reach high enough DT with light armor to shrug off most attacks, so there was almost no point in dealing with heavier armors for all of their drawbacks.

Morrowind was not DT.

The problem you describe is an issue of balance, not the system itself. Oblivion had the exact same problem and used a simple X armor = X% damage reduction system (capped at 85). Heavy armor had 50% more armor than light armor, but the master perk for light armor gave a 50% boost to armor value. You'd have the same amount of armor with none of the drawbacks. The problem isn't inherent to the Damage Threshold idea.

[EDIT] The system employed in FO4 seems to solve that problem. I just wish it were more mathematically comprehensible.

Dude, tip; dont get it if you want to use power armour because lategame you'll rarely be in need for power cores so you'll be in them constantly.

To compare; best PA upgraded to its max can take ATLEAST 2000 ballistic and energy, depending on setup.
For 5 perks you get 50 DR.. Which, to me, is nothing and basically wastes 5 levels