I think it will have something to do with your guardian's subclasses and the element they carry (solar, arc and void) so far we saw 2 subclasses per guardian with a certain element attached to it. + it also seems like there is a 3rd subclass slot.... I wrote a thread yesterday about this and a kind fellow pointed something out, wich leads me to speculate that we have yet to see: A AoE Void Hunter, A Mellee Arc Warlock and A Solar Titan. Disclaimer: this here is both a possible answer and a question at the same time!