Flat RR applies once(highest value). If you have in theory 20 rr all and also 30 rr to physical resistances then all wil be reduced by 20 but phys rr by 30. So Elemental Stom will Override Decree of Aldritch in terms of elemental RR but decree of Aldritch will RR everything else like Aether, chaos phys, vit, pierce etc.
-% RR Stacks if from Different Sources. Widow will Stack with Vulnerability in terms of lightning RR or with Warpfire in terms of -% Aether.
%RR Applies once and last of the three. Highest value, same rules as flat RR
-X% <type> resistance (Arcane bomb etc)
This one stacks from all unique sources. This can bring mob’s resistance negative.
X reduced target resistance (Acid spray, Decree of Aldritch, black star of deceit etc)
Does not stack. The highest source will supercede any. This is a flat amount and can bring mob’s resistance negative.
X% reduced target resistance (Open hand of mercy etc)
Does not stack. The highest source will supercede any. This is a multiplicative effect and is calculated last after the first 2 types are affected and therefore cannot bring enemy resistance to negative.
Example calculation mob has 100% aether resistance and you have 50% of (3) property. After say -20% from arcane bomb (1), -15 flat from (2) leaves behind 65%. Reducing it by 50% would give you 32.5% final Aether resistance.
In the event that you have more than 100% total RR affected on mob, each source will be reduced according to weightage.