Control is a state which magicians have to learn to be able to control their magic. If they lose control or do not master this stage they can die as a result. They unintentionally release magic and destroy their surroundings.
Control is taught to Guild novices after their magic has been loosed, however naturally strong magicians like Sonea can loose it on their own. People with latent magic can also loose their own if they know how. However natural magicians must learn control because their powers are stronger than most magicians, and so when they release magic by accident it is more dangerous.