Agony resistance cap.
The Defensive or Offensive Infusion slot can take a +5 AR such as the Versatile infusion that you buy for 75 relics.
The other slot that is added for an infused ascended ring can take the Agony infusions that you combine the +1s from. The max was put out to be +30 AR for an infusion by Anet when it was first released I think. +15 is the highest that someone has posted to the TP so far though.
+5 AR x 6 Trinkets = 30 AR
+5 AR x 6 Armor Pieces = 30 AR
+5 AR x 2 Weapon Slots = 10 AR
= 70 AR
The maximum Agony Infusion that can be made right now is a +16, even though a +30 exists in the dat file. Thus, for the whopping price of ~2000g each, you can add:
+16 AR x 3 = +48 AR
= 118 AR
Don’t forget the back slot as well. I’m running 83 AR right now and have open slots on my ascended infused back slot yet. Remember to deck out both sets of weapons incase you switch in combat. I’m curious to see what new infusions they will have with HoT.