Eliquo is the flow of energy between two realms. The more focused an Angel is in the eliquo between it and a called upon spirit, the greater the spirit's effect upon the Angel will be in the mortal realm. Think of eliquo as a flood gate. The more open the gate, the more energy a Guardian Spirit may channel through. Note: Unlike the other disciplines, eliquo is an integral part of the spirit calling process. Therefore, it will naturally grow on its own, and an Angel need not focus upon it, unless they should wish to.
Plain speak and speculation:
Eliquo is one of the disciplines available in every Guardian Spirit. As the description above indictes it is integral to each spirit. in fact your rating in a spirit's eliquo seems to impact how effective every related power is.I've also noted that, at lower levels, essence regeneration seems to be quite slow and I ran out often. Because it is such an important discipline, it gains gxp at all times, even when you are currently focused on leveling another discipline within that spirit.
From my own personal experience, Eliquo seems to be a spirit's "Mastery" just what effects it has over slight improvements to the subdisciplines is unknown, though I speculate one benefit might be the improvement of a spirit's passive effects.