Resonance
Resonance refers to a person's natural ability to channel magical energy. The entire population of Sharum Mal'Kari is believed to have at least some traces of Resonance within them. However, the level of Resonance an individual has varies greatly from person to person and appears to be related to genetics, with certain bloodlines seemingly predisposed towards having particularly high (or low) levels of Resonance. In the case of malaks, Malakim, and Fey few things go further towards establishing their formative identities, as resonance is largely what determines which artes they use. No matter how anyone may try to alter their Resonance, Resonance is believed to be innate and therefore unable to be changed by training.
Generally speaking, individuals with higher Resonance are seen as more powerful than their peers, and may exhibit a variety of natural magical talents. It is usually easier for them do things that rely on magic to function, such as utilizing artes, connecting to the Primordial Sources, or making Pact with magical entities.
Resonance plays an even larger role in Fey society, with high levels of Resonance an expectation of (and unofficial prerequisite for) nobility. It is distressingly commonplace for Fey royalty to disown their children that have not demonstrated a sufficient level of Resonance, and to adopt others who show strong natural talents. Equally, it is completely unheard-of for Fey to follow the leadership of someone with low resonance-- which is one reason why Fey often view societies outside their own Courts with great skepticism... and disdain.
See also: Artes