The elements of the subclasses have been referenced as being the fundamental forces of nature in the Grimoire. So there's gravitational, electromagnetic, strong nuclear, and weak nuclear. Arc is electromagnetism, Solar is strong nuclear, and Void is most likely gravitational. This would make a lot of sense as a Nova Bomb is this large mass dropped upon your enemies and the Nightstalker arrows slow enemies or, in a sense, increase the gravity imposed upon them.