Each of the given accessory slots has three different sorts of Augment that can be applied; one adds boss damage and critical chance, one adds critical chance and critical damage, and the last adds HP and PvP defense. Each sort has a maximum amount that it can add to an accessory, but each augment application is randomized; you could wind up with the best values right away or roll them multiple times without much luck. Check out the full rundown for numbers before you get ready to make your best accessories even better after all.
Source: Official Site