Unspecified damage type

[Yak folklore]

Based on my understanding, unspecified is essentially its own type of damage, but how does it work? Does it count as every type of damage when it comes to damage specific DR? does it count as none of the damage types for resistance? Does it do normal damage to creatures with regeneration? I am also just curious if there are any non magical sources of unspecified (and non-obvious) damage.

In case it matters your answer I am specifically thinking of a ghost's corrupting touch, which I always assumed was negative energy damage, but it doesn't say that.

[Venger]

No.

Untyped damage is exactly that: untyped. "Untyped" is not a type of damage, which is why you don't see spells giving "untyped" resistance 10 or monsters with "dr 10/untyped." Damage from spells always ignores dr, regardless of the type. Untyped damage is not any type for the purpose of resistance, yes. No, only the types of damage called out in a creature's regeneration will deal lethal damage to it. For example a troll has regen/fire or acid, so untyped damage would not deal lethal damage to it. There are plenty of nonmagical sources of untyped damage. A ghost's corrupting touch is untyped damage. Nothing is negative energy damage (or any other type of damage) unless it says it is.

[Yak folklore]

Corrupting touch is an SU ability, I mean truly non-magic, but thanks for the rest.

[The Viscount]

There are poisons that deal hitpoint damage; this damage would be considered untyped (though creatures with immunity to poison would be protected against it).