Magic is energy. To best show my thoughts, think of magical energy as water. There is water within the air around us, this would be Ambient Energy. There are also lakes, rivers, streams and oceans of water which, would relate to streams of magical energy more commonly called Leylines, Magical Pools , Nodes and what not. They, as their water counterparts, would all vary in strength and size going from a babbling brook to a raging river to a calm lake to the wind tossed sea.

When a caster uses a spell, they release the spell energy for a desired effect. For example, a fire ball. Magical energy is released in the form of a fire ball. When the fireball is completed, what happens to the magical energy that went into it? Part of it is transformed into fire and part of it will slip away into the ambient energy. In an area where several spells have gone off as in a fight or magical duel, there may very well be Residual Energy that could remain there for a long while. Much like after a hard rain, there might be large puddles of water that slowly dry up. So too would it be with magical energies, I believe. They would pool in one place and slowly drift away into the ambient energy till there was nothing left.

A caster who draws their power from a natural source of energy such as a lake or a volcano would perhaps be a specialty case. As for classification, there are different degrees for these types. Meaning a caster who has just discovered they can draw magical power from a lake will not be as strong as say someone who has been doing it for decades. As with people, there are many different shades of magic, each varied in their workings. It could perhaps be said that a beginner in a field (i.e. an Apprentice) could only draw small amounts of energy, or only do so once a day even. Where as a Master would pull more as well as more often.

As for role playing, perhaps a caster of this type of magic would call themselves an Energy Magician/Magi/Adept...etc. There would be different teaching and so forth depending on the type of study and technique used... such as "I am Migan, the White Winds Magician". White Winds is a Mage school which I pulled from a series of books I have read. Different schools may have slightly different scales they follow as far a ranking goes.

There would also be natural nodes of energy. By nodes, I mean a gathering place for energy or a place where two leylines (streams/rivers etc..) of magical energy cross, much as rivers and streams flow to a lake or ocean. These nodes (lakes) of magical energy could be tapped into and used by a caster with the sufficient strength and knowledge to do so.