and this「Spend an Insight to:● Add +1 AD to a successful attack roll as critical damage.」
Does this mean an extra 1D6 damage, +1 damage, or throwing another AD to try may be available more damage?
Spending an Insight on an attack adds an AD to the attack after the fact. Since damage is calculated as the difference between the highest AD and the highest CD, this means spending an Insight can be compared to landing a perfect parry in a Soulslike video game.
For example, if a character strikes an enemy and rolls a 5 on their AD versus a 4 on the opponent's AD, they're only dealing 1 damage. If they spend an Insight, they roll the additional AD and potentially deal 2-7 damage with the attack instead.