This is a 5% increase to your current parry rate.
Another example is with the job ability Battuta. This gives a 40% increase to your base parry rate, and with each merit point upgrade into this ability it increases in 4% increments.
Additionally, the effect from the job ability Battuta and the job trait Inquartata aren’t determined by the enemies’ strength, but applied directly as a percentage. (In cases where you see "Parry skill" and "Parry skill +", these values will change depending on an enemies’ strength.)

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