All humans are born with energy channels in which energy flows through them. Each human has a specific type of elemental energy that he or she is born with that can be used to perform elemental attacks and magic.
Though everyone is born with a specific type of energy, not everyone is capable of accessing those energy channels in order to utilizie it. Magicians are particularly skilled in accessing these channels, as their attacks are powered by their energy. Mercenaries are trained to be able to tap into this extra resource in their bodies.
It is possible to acquire different elements within one's own energy channel, but it requires extensive training and is usually only possible for mages. The downside is that one must be very careful to maintain complete control over all elements running through their energy channels through meditation and other focusing techniques, or else the energy may be come corrupt and destroy the body.