There are two related rules concerns that the community needs official guidance from Paizo to resolve. Both of these rules are ran differently at various tables, and the community cannot agree on the RAI since Player Core contradicts itself on Page 408.
Rule concern 1) Please define the phrase "instance of damage."
This phrase is only used two times, both times on Page 408, once under Weaknesses and once under Resistances. Unfortunately, Paizo has never defined this phrase. The two interpretations that the community uses for Instance of Damage are:
Definition A) A single strike/spell/ability that deals damage deals a single instance of damage. (i.e. a Flaming Longsword that deals 1d8 Slashing + 1d6 Fire is ONE instance)
Definition B) Each damage type dealt by a strike/spell/ability (i.e. a Flaming Longsword that deals 1d8 Slashing + 1d6 Fire is TWO instances)
For normal cases, everyone typically uses definition A, resulting in Weakness/Resistance only being applied one time per strike/spell/ability. The community (and PFS tables) become split when Holy/Unholy traits are brought into the equation, which leads us to the main contradictions we need Errata for.
Rule Concern 2A) How Weakness to Holy/Unholy is calculated against multiple damage types.
Consider a demon with the Unholy trait has a Claws Strike that deals Slashing + Spirit + Poison damage. The demon attacks a PC with Weakness 5 to Unholy. The relevant rule from page 408 says "If more than one weakness would apply to the same instance of damage, use only the highest applicable weakness value. This usually only happens when a creature is weak to both a type of damage and a material or trait".
Weakness 5 to Unholy is ONE weakness, not "more than one weakness," so technically that rule doesn't even apply. Assuming is does apply though, we need a definition for "Instance of Damage" (See A or B above) to rule how to apply the Unholy Weakness; should it apply one time to the whole Strike, or three times - once for each damage type within the Strike?
The ruling I see most often for the Holy/Unholy Weakness is Definition A. However, the community primarily uses Definition B when dealing with Holy/Unholy Resistances!
Rule Concern 2B) How Resistance to Holy/Unholy is calculated against multiple damage types.
Consider the same Claws strike from the demon described above, hitting a PC with Resistance 5 to Unholy. There are two relevant, contradictory rules under Resistances on page 408, each one lending itself to a different definition of "instance."
The first rule supports Definition B of "instance": "If you have resistance to a type of damage, each time you take that type of damage, reduce the amount of damage you take by the listed number (to a minimum of 0 damage)." In this scenario, the Resistance 5 to Unholy would apply separately to all three damage types the demon deals. This is the same way that "Resistance to All" damage works.
Rule 2 supports Definition A of "instance": "If you have more than one type of resistance that would apply to the same instance of damage, use only the highest applicable resistance value, as described in weakness." In this scenario, the Resistance 5 to Unholy would only apply one time to the strike.
From what I can tell, the majority of the community uses the Rule 1 / Definition B interpretation for Resistance to Unholy, but uses the Definition A interpretation for Weakness to Unholy.
To fix these rule contradictions, we need errata the defines what an "instance" of damage is (Definition A or Definition B), and reiterates how the Holy/Unholy traits should apply.
Thank you for you time and consideration.
