Hafling Keen Eyes (Ancestry Feat)

[Xiphose]

it reads

Your eyes are sharp, and your other senses are
similarly attuned to your surroundings. When you
attack an opponent that is concealed from you or sensed by you,
reduce the flat check DC you need to hit that creature to 3 for
concealed or 9 for sensed. You gain a +2 circumstance bonus
when using the Seek action to sense unseen creatures within
30 feet.

Do I just reduce their AC to hit by the amount stated?

like if a monster has cover I just take away 3 from their AC to hit?
and consequently 9 for invisible creatures?

Dont really understand

thanks c:

[Kennethray]

In the concealed rules, or sensed rules before you attack you have to make a flat check (roll d20, add nothing) to see if you are even able to hit them with that attack. I may be off on the timing but that's the jist of it.

K-Ray

[Draco18s]

Xiphose wrote:

When you

attack an opponent that is concealed from you or sensed by you,
reduce the flat check DC you need to hit that creature to 3 for
concealed or 9 for sensed.

Bolded the relevant part. You reduce the flack check (that is, PF2's version of "20% miss chance due to concealment"), as Kennethray says.