UAX is an acronym derived from the term UAS (Unlock All Skills). UAS means that you have unlocked all the skills in all of the campaigns you own, for use on your PvP characters. UAX is a step after that. The X can be anything, much like the common variable X in programming and mathematics. This means that if you have UAX then you have unlocked all the skills, runes, weapon upgrades, inscriptions, and heroes in the game.