I've noticed this while playing through that there seems to be different enemy tiers of difficulty. Take something like soulfire plateau in Auridion, there are three witches around level 7, each with two pins (...I'm not sure what to call them) on their healthbars. Normal trash mobs don't have this, commanders or trash+1 usually has a single tag.

Has anyone given names to these "tiers", how high do they go and is there a list somewhere I can look at?