I'm not sure how it works. I gave it to Xoti, who is wearing a fine breastplace, with no fire protection listed. But when Xoti is wearing it, she has now 11 armor burn rating, despite the ring only giving 2 in its description and the armor not being specific for fire damage.

I'm reading the game's rules wrongly, or it's the ring?

...Say, if an armor reads "Armor Rating 7", that means it has that armor rating of 7 against EVERY type of damage, elemental included. Then there are the weak points, usually one physical and one elemental, which are lowered below that 7. 

 

Your breastplate has a default armor rating of 9. With +2 from the ring, 11.

Thanks for the info