Rank defines an NPC's difficulty based on its rating and level. Rank is divided into 6 classifications: Novice, Disciplined, Seasoned, Expert, Veteran and Master. They are indicated by a dot system on each NPC's health bar. Each level of rank should be more difficult than previous levels. If you take two mobs of the same level and rating, the one with a higher rank will be more difficult to kill. Higher ranks usually include higher attributes and skills than lower ranks.