I’m sure somebody will correct me if I have this wrong, as I know there were some nuances in NWN2. However, I believe it is as follows …
Damage Reduction reduces damage versus normal weapon damage by the amount stated, unless a weapon meets the “pierce” type required. So if a +1 weapon is required to overcome (“pierce”) the damage reduction, then such a weapon used does full damage. i.e. It has overcome the reduction and so does full damage. If not “pierced”, then the damage is reduced by the said amount.
Damage Resistance simply reduces the amount of damage of a particular type taken. Type taken refers to types like bashing, piercing, acid, fire, electric, etal. So, if a creature has Damage Resistance 10/acid, then any acid damage done to it is reduced by ten points. So at least 11 points of damage is required to do even one point of acid damage. NEEDS CHECKING: If I recall correctly, additional elemental type damage may only be added if normal damage does any damage … but I may be thinking of pen and paper application only and NWN2 may just do it.
Damage Immunity … Assuming you mean, EffectDamageImmunityIncrease, creature has damage taken decreased by immunity type percentage. E.g. Immunity increase 50% v Acid damage would have any acid damage reduced by 50%. Therefore, 10 acid damage would be reduced to 5 damage for this creature. (Limited between 5 - 90%)
Damage Vulnerability … Assuming you mean, EffectDamageImmunityDecrease, I have no experience with using this function. However, I imagine it works the opposite way as the function above, and would add a percentage of the original damage. E.g. Immunity decrease 50% v Acid damage would have any acid damage increased by 50%. Therefore, 10 acid damage would be increased to 15 damage for this creature. (Limited between 5 - 90%)
As a side note, the only way to apply a full immunity to damage (100%) is to make a creature plot or immortal.
I also copy an old post that may be of help to you regarding Damage Reduction below.
DAMAGE REDUCTION RESULTS
NB: Since this post was originally made, please bear in mind results may differ depending if the effect is used on a PC or a non-PC creature.
EffectDamageResistance: Cannot use this function to add ANY DR reduction. (It is used with ELEMENTAL TYPE DAMAGE.)
ItemPropertyDamageReduction: Have not been successful in adding DR to skin or armour items.
EffectDamageReduction: Have only be successful in adding a pair of DR and not one by itself.
(Use IP_CONST_XXX and reverse logic.)
IP_CONST_DAMAGETYPE_BLUDGEONING = 0
IP_CONST_DAMAGETYPE_PIERCING = 1
IP_CONST_DAMAGETYPE_SLASHING = 2
eDR = EffectDamageReduction(5, 0, 0, DR_TYPE_DMGTYPE); // Applies DR for P and S (B Gets Through)
eDR = EffectDamageReduction(5, 1, 0, DR_TYPE_DMGTYPE); // Applies DR for B and S (P Gets Through)
eDR = EffectDamageReduction(5, 2, 0, DR_TYPE_DMGTYPE); // Applies DR for B and P (S Gets Through)
SET VIA TOOLSET PRIOR PLAY (CREATURES AND ITEMS): Appears to work if you bear weapon corrections into account (See next).
The Morningstar is BLUDGEONING only (TLK 5412 NEEDS CORRECTING) and that the Longsword is both SLASHING AND PIERCING!
Also SLASHING & PIERCING: Bastard Sword, Halberd, Katana, Scythe. (Total five weapons.) Therefore, Longsword (TLK 5417), Bastard Sword (TLK 5434) and Katana (TLK 5423) need descriptor correction.
From the toolset setting, we have the option of applying “OR” logic to the “piercings” selected, which means we can make it so a creature (or item) has DR against only ONE type. In theory, as applying this to a “skin” item is possible, it means we can also create different skins and alter “monsters” on the fly by swapping out “skins”. NB: Using skins with PCs is NOT possible in NWN2 due to problems that come from doing so. (See my blog for details. Search for “skin”.)
For PCs, a better solution would be to use the DAMAGE IMMUNITY (5-90%) v WEAPON DAMAGE TYPE rather than the DR for an item when only one DR type is required, due to the problems noted in the image in the previous post. (i.e. Property presentation and lack of recognising the property is actually present!)
Weapon (Type) | v DR Type (“Pierce Required”) | Bypass Y/N
Longsword (SP) | BSP | N
Longsword (SP) | B | N
Longsword (SP) | S | Y
Longsword (SP) | P | Y
Longsword (SP) | BP | N
Longsword (SP) | BS | N
Longsword (SP) | SP | Y
Club (B) | BSP | N
Club (B) | B | Y
Club (B) | S | N
Club (B) | P | N
Club (B) | BP | N
Club (B) | BS | N
Club (B) | SP | N
Spear (P ) | BSP | N
Spear (P ) | B | N
Spear (P ) | S | N
Spear (P ) | P | Y
Spear (P ) | BP | N
Spear (P ) | BS | N
Spear (P ) | SP | N
Halberd (PS) | BSP | N
Halberd (PS) | B | N
Halberd (PS) | S | Y
Halberd (PS) | P | Y
Halberd (PS) | BP | N
Halberd (PS) | BS | N
Halberd (PS) | SP | Y
Morningstar (B) | BSP | N
Morningstar (B) | B | Y
Morningstar (B) | S | N
Morningstar (B) | P | N
Morningstar (B) | BP | N
Morningstar (B) | BS | N
Morningstar (B) | SP | N