If I remember right, DR/- means that nothing overcomes that DR. However keep in mind that DR is only for weapons, not spells. You need SR (spell resistance) for spell defense.
The detail that comes after the "/" in a damage reduction statement says what kind of weapon can overcome it.
DR x/Magic can be overcome by any weapon that has a magical enhancement bonus.
DR x/- cannot be overcome by any type of weapon. (Exception: Paladin Smite Evil ability.)
Neither of these have any impact on spells or magic unless the spell specifically calls out a damage type (such as 3d6 bludgeoning damage). There is still a question vaguely out there as to whether DR x/magic would work against a spell that does (to use the same example) 3d6 bludgeoning damage. A fireball ignores both types of DR completely.
Dr/ magic is overcome with a magic weapon.
Dr/- is not.
Spells are not effected by dr.
What about Adamantine weapons? We're using Armor Bonus to DR alternate rules currently. As seen here on Table: Natural Armor Conversion to DR, normal Natural Armor is supposed to be DR/Magic. However, DR/Magic converts to DR/Adamantine. DR/Adamantine grants DR/-.Where is material required to overcome Natural Armor mentioned? For example, a Balor has Natural Armor of 16. Does that mean his Natural Armor is normal Natural Armor, which means it grants DR/Magic?
I am actually pretty confused.
You're misreading that.
This is not a regression. Only convert one time. Or in other words, only convert a normal monster to a Armor-as-DR monster. Never take a monster that is already converted to the Armor-as-DR system and then convert it again.
Also, its Natural Armor and Armor bonuses become "DR x/Armor" which is a new type of DR just for this alternate rule.
If a creature has normal natural armor, say, like an Ogre (who has Natural Armor +5), then the conversion means he gets DR 5/armor. Period. Stop there. Do NOT convert that into DR 5/Adamantine because you have already converted this ogre to the Armor-as-DR system once, so don't do it twice. The ogre also has normal armor (hides) that give it an Armor bonus of +4, which also converts to the Armor-as-DR as DR 4/armor. Adding these together I get the converted ogre to have a Defense of 8 and a DR of 9/armor.
Sadly, this gives it an equivalent of DR 9/Magic, which seems to mean that any schmuck with a +1 dagger(or any other +1 weapon, or a non-magical weapon with the Magic Weapon spell cast on it) can totally ignore the ogre's entire 9 points of DR, will only need an 8 to hit the ogre, and will do full damage to it as easily as if the ogre had no hide armor and skin as thin as a weak halfling...
(I admit, I don't use this system, but that seems to be really weird to me, that all that armor can be so easily ignored - maybe I am just misunderstanding. In any case, I'm fairly sure the part about not converting and re-converting the same monster is right)
A spell that deals bludgeoning damage would get reduced by DR.
Although the Bestiary definition of Damage Reduction (page 299) says "The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities," that's actually just referring to damage that isn't specifically called out as being of a particular type, such as fire damage or piercing damage. In other words, DR doesn't protect against "typeless damage" from magical attacks.
However, if a magical attack specifically mentions that it deals bludgeoning, piercing, or slashing damage, DR affects that damage normally, as if it were from a physical weapon. (Otherwise the magical attack might as well not have a damage type, as it would only interface with B/P/S damage in a very few corner cases, such as whether or not an ooze splits from that attack.)
For example, the ice storm spell deals 3d6 points of bludgeoning damage and 2d6 points of cold damage. If you cast ice storm at a group of zombies, the zombie's DR 5/slashing protects them against 5 points of the spell's bludgeoning damage. Their DR doesn't help them against the spell's cold damage because DR doesn't apply to energy attacks.—Pathfinder Design Team, 03/06/13
If it is DR\X and Y, then your weapon must be both X and Y to bypass it. If it is DR\X or Y, then your weapon can be either X or Y to bypass it.
Oh wow. I haven't even noticed those differences in the wordings.
I was really stupid.Thanks for clearing it out for me. I feel like an idiot now.
Don't feel bad. The reason I know? I made the same mistake just a couple months ago, heh.