idk how other way i can tell you that ac in bg3 is nonsense and work only as dodge mechanics
In the game of Dungeons and Dragons (which BG3 is loosely based on), you have Hit Points - this represents several things at once; it's both your physical health, but can also represent your stamina, your morale and willingness to fight, and several other things. Most commonly, however, it represents you
Capability to fight, however that is envisioned by you and your DM.
When hostile creatures attack you, they are generally attempting to do you harm and reduce you capability of fighting or resisting; often they are outright trying to kill you.
When a creature attacks you, be it with a melee weapon, magical spell, ranged weapon or some other means, they are usually called to make an Attack Roll - this roll represents, in the abstract, their attempt to strike you and cause you harm; it represents
both the the creature's ability to counter your defences, and also their own accuracy and capability to land the blow at all.
You defend against this attack with you Armour Class, which represents both your ability to avoid the attack, to whatever degree you can, and also to block, deflect or divert it, if possible, as well as any other way that you might avoid harm - such as having cover from a wall.
This is a binary check, yes! If the creature's attack roll with all modifiers meets or exceeds your AC with all modifiers, then the creature has succeeded in causing you
Some degree of harm, in
Some way. If they
don't then it means that,
for whatever reason, they have
failed to cause you sufficient harm enough to count as even a single point of damage.
Okay... so now we know whether they've managed to hurt you at all; perhaps they rolled a 2, and perhaps their failure is quite literally that they misfired their bow and the arrow never even reached you. Perhaps they rolled well, but your AC was still higher, and their arrow ricocheted off your raised shield. What matters is that in a failure situation, they were unable to strike you sufficiently well to cause even a single point of damage.
Doesn't mean they didn't hit you, necessarily - just that
they didn't do it well enough to cause any harm.
Suppose they rolled well enough to hit you - this doesn't mean they necessarily scored an incredible blow that you mitigated none of the damage of whatsoever; it just means that they
succeeded in hitting you well enough to cause
Some degree of harm. How well did they hit you? How much damage did your armour protect you from, and how much energy of the strike did you divert or deflect, or did you partially managed to pull yourself out of the way of, or make less severe? Well, we work that out in the
Damage step.
In the damage step, the attacker rolls to see how much damage they dealt. If they roll high, that usually means they scored a solid telling blow, and your defences, whether avoidant or blocking, weren't able to protect you very well in this instance - perhaps they found a gap in your plate, or perhaps the blow simply had enough impact, and landed cleanly enough to wind and harm you severely through it. Perhaps they rolled a 1 on their damage die... in this case, while the enemy did manage to hit you well enough to do harm, your armour absorbed most of it, or you moved enough with the attack and your armour deflected most of the rest; you still got hurt, a little, but in reality, you only suffered a graze, because your defences - whether armour or otherwise, helped protect you from the majority of the harm.
That's how it's rationalised in THIS system - that's how amrour is conceptualised as reducing damage and protecting you in THIS system. It makes sense within this system; no it's not 100% realistic - it's deliberately abstract to allow for a lot of flexibility and interpretation between players and DMs.
