Fate is one of the three axes of magic (along with Erf and Numbers) used to break a major class of magic into three different disciplines. Erf-b1-p038a Some Erfworlders also use the term "Fate" to refer to the predetermined destiny of an individual or group, as if it was a magical Stat.
It is strongly suspected that Fate provides the strings by which the Titans enact their great plan, subtly manipulating events in an effort to reach a desired state.
Fate views or changes the purpose or destiny of whatever it deals with. For items, that is what it does. Turnamancy can take an existing object and make it work itself (adding Motion), but to take something and give it a purpose beyond what it has innately takes Dollamancy. When one form of regular Stuff is changed into another by a Changeamancer, it’s Matter and Fate. Healomancers directly modify the Fate of whatever they cast on. Fate Magic is more subtle than Erf magic, but less so than Numbers. Erf Magic has an effect that is immediate. Fate Magic tends to be more long-lasting. Changing the nature, purpose or direction of an object is a profound thing and that’s what Fate does.
The Axis of Fate seems to focus on an active target's status. For instance, Croakamancy can return a character from croaked to uncroaked. Dollamancy (and by proxy fabrication) specializes in the creation or re-configuration of a base material into non-living items like raiment, golems and small, non-magical items. Presumably, Healomancy restores hit points and removes other status effects. Thinkamancy is the guiding magic behind a unit's decisions and has been shown to alter loyalty scores. Predictamancy can look into a target's future state - alive, dead, carrying around an Arkentool. And Carnymancy can manipulate the very rules by which the world operates.
Signamancy is the Fate magic that forms the bridge between the Fate axis and the Erf axis. While Fate works as Erfworld's control and logic mechanism, Signamancy is used to shape the physical erf into a symbolic, visual representation of what happens behind the scenes.
It has been stated that a price must be paid for every transaction, including very powerful fates. Wanda has been described as very special within the Fate axis, and Delphie Temple said that she had had to pay out a debt to Fate by working for Haffaton. It is likely that there is some sort of "karma" score to balance out the good and bad events fated to happen to a unit.
Loyalty may be considered a form of natural Fate magic, in that being loyal to a side is aligning ones fate with it. This is probable, as Loyalty and Duty are described as natural Thinkamancy.