There are many attack type:
1- when your hero attack whitout item.
2- if the hero attack whit a auto cast skill like fire arrow...
3- if you use an orb ( exemple orb effect dosent stak, because they got diferent attack type, same thing whit skill like fire arrow, if you use an orb you lose the effect or the fire arrow because they got diferent attack type )
4- execption: if you got a skill like frost attack or envenemed weapon... no problem whit them, the effect stak because they are passive skill.
5- a special one: the Mask of Death, this is a attack type to like orb, dont confuse it whit Vampiric Aura, the Mask of Death is an attack type not an aura so it work on ranged hero. Now the special part whit this attack type effect, the Mask is like an orb, but, i dont remember on whitsh patch but they mod it to make the life stealling effect able to stak whit an orb.
so remember dont fil your inventory whit 6 diferent orb or the only thing you can get is the damage bonus, and if you got lv 3 fire arrow on your pristess leave the orb for an other hero