I'm pretty sure Unique means that there can be only 1 copy of the character/monster in play (you discard the one already in play and put the new one). One example is Redcloak, which has a regular monster version and a special Xykon's lair one. This rule was also to address the Linear Guild expansion (when it was eventually released) which would come with copies from both playable Linear Guild and BattleDeck OoTS.