Rarity
Creatures, items, traps, hazards, special abilities, and many other things can be common, uncommon, rare, or unique.
Attempting to learn information about uncommon, rare, and unique targets have DCs that are 5, 10, and 15 points higher than normal (respectively).
The first time such abilities are used against you, they likewise apply these bonuses to their DCs and attack rolls. These bonuses are reduced by 1 each additional scene you're exposed to them but completely negated once you succeed in a knowledge check against them.
If you have a specialty against something or successfully succeed in an Analyze attempt against it, you treat the target as one degree less rare than it actually is, to a minimum of common.