Well, basically, the White Wolf games (of which Mage is one) are built on a system called the "Storyteller System" which is unique in that it doesn't utilize D20's, or any dice other than a dicepool of multiple D10's. Because of the name Storyteller System, people who run Mage (and other) White Wolf games are ST's instead of GM's or DM's. I currently run 2 separate Mage: The Awakening games on Roll20... I am pretty sure that I am the only Awakening GM. I know of only one Ascension GM as well.