While Will users can accomplish amazing feats their specialty would be their Sovereign, Sovereign means supremeacy over something or someone and in a Will users case it means specialty over a specific type(s) of Edicts. For example, Sovereignty over water would make all edicts that manipulate that element more potent. A persons Sovereignty is unique to their biology and soul meaning that they usually have a Sovereign that correlates to their species, enviorment, and history but having its not always the case. One cannot choose what their Sovereignty is but with enough practice they can expand it or even get another one. 

For example, a race of beings that live in the desert would most likely have sovereins pertaining to something about the desert such as manipulating sand or heat. The opposite of that would be water and cold and using Edicts of that type would be harder and less effective. However, if the person had water Sov's in his bloodline he has the chance to have a water Sov as well.