Fates are agreed upon by oracles from every planet, known to be true no matter what circumstances precede them. While many visions seen by oracles are known to be only possible futures, fates are considered set, and cannot be changed. Thus, there are many different ways for a person's fate to come to pass, and fates are therefore a bit vague in language.
"Nine lines of nine families, spread across the galaxy, their members' paths predetermined and unalterable."
Not every family is considered "fated," meaning, there is nothing specifically set about their futures during the course of their life. It is also indicated that fated families are considered to have a more meaningful path, indicated in part by the fact that chancellors from Thuhve are not elected by popular vote, but instead fated to be. Likewise oracles are known to be fated. Most likely because of this, peoples' fates are kept incredibly private. Oracles and Assembly Leaders do not reveal the fates (with the notable exception being the beginning of Carve The Mark) and they are forbidden to be discussed among the Shotet people.