Shamans are an alternative path towards magic power, quite different from Rune Priests and Rune Lords. To begin with, many shamans are outside of the structures – they are initiates of the Horned God. They often operate adjacently to the cults, establishing up spirit cults or lead ancestor worship.
In many cultures (like the Orlanthi), they are a magical outlet for people with magical power that do not the conventional cult structures. Their powers typically come from their shamanic initiation, and not from a god. Initiates are rarely prohibited from becoming shamans, but nor are they encouraged.
A few notable exceptions exist. Waha‘s priests are shamans, as are those of Daka Fal and Jakaleel. Yelm even has a shamanic order, that of the Golden Bow, descended from the original solar priests of the Pure Horse People.
But in general, shamans are ambivalent figures in most cults. Many prefer to avoid the entanglements and demands of the gods. They are beloved for their ability to deal with hostile spirits and to provide the blessings of spirit magic. Feared for their outsider status. They tend to have more relationships in the Spirit World than the mundane world.