Exceptional bonus

Exceptional bonuses are very rare bonuses generally only found on high level loot. Exceptional bonuses stack with everything except effects with exactly the same name. Note that with Update 14, the developers updated most exceptional bonuses, and in most cases changed them into the non-stacking insight bonus instead. However, some enchantments were not renamed (EG: Exceptional Fortification), but their actual bonus type was changed. This page lists all known exceptions to that change, so you can assume all other exceptional bonuses are now typed as insight bonuses. The developers have stated that they prefer to offer non stacking bonuses to create more gear options and so do not plan to add more items with exceptional bonuses in the future.

Exceptional (Stat) Skills +1, +2 and +3
These bonuses exist for each and every stat and effect all skills that use that stat as their modifier. They only appear on named loot, most notably Green Steel accessories.

Example: An "Exceptional Charisma Skills +1" bonus stacks with an "Exceptional Charisma Skills +2", but not with another "Exceptional Charisma Skills +1" effect.

Previous Exceptional Bonuses
Prior to Update 14 there were also exceptional bonuses to fortification, sneak attack, and elemental resistance. These were all changed to insight bonuses, except elemental resistance. The +5 versions of elemental resistance were changed to competence, and the +10 versions were changed to insight.