[0-70[ : 2AC = 1%
[70-140[ : 3.5AC = 1%
[140-210[ : 5AC = 1%
[210-280] : 6.5AC = 1%
AC still caps at 280.
In live server, every 3 ac reduces damage by 1%.
I'm not sure how the system differentiate between physical and magical damage reduction.
From what I understand, all AC type i.e. white, blue and red, reduce physical.
Only white AC reduces magical damage.
Let's see what is the impact of the AC change in PTR via direct calculation.
Live server AC system (1% damage reduce every 3 ac):
0 ac - 0% damage reduce
70 ac - 23.33% damage reduce
140 ac - 46.67% damage reduce
210 ac - 70% damage reduce
280 ac - 93.33% damage reduce
PTR server AC system:
0 ac - 0% damage reduce (no change from live)
70 ac - 35% damage reduce (buffed from live by 12% increase)
140 ac - 55% damage reduce (buffed from live by 8% increase)
210 ac - 69% damage reduce (nerfed from live by 1% decrease)
280 ac - 79.77% damage reduce (nerfed from live by 13.57% decrease)
In summary, up to 202 ac your damage reduction will receive a buff compared to live server.
At 203 ac, your damage reduction will start to get nerfed compared to live server.