According to some info on SteelWarriors site, Avoidance and combat effects cap, and it appears to be a hard cap, at +120. It's possible Accuracy may cap at +120 as well as it appears to be of the same time of Mod.
Avoidance adds to your raw chance to avoid incoming damage as in dodge. Seems every +10 = 1% chance. and max at +120 is a +12% raw dodge chance. They're not sure what order it adds its effect, but its effect is extremely nice for the classes that get beat on a lot such as warriors, Monks, Chanters, Shaman and clerics. Its effect stacks with Imp Dodge 3, +% to dodge skill, and Talisman of sense buff.