It appears that High Magick has to do with Universal perspectives; thoughts on Gods personality, reasons for existence, Universal Order. Low Magick is about personal want and Will, like forming spellwork to get what you desire in the immediate; healing spells, love spells, revenge spells.
In High Magick the magician ascends and actually becomes a Demon where in Low Magick the magician calls upon them. The highest Demon is God.
All Demons are born of Chaos except the Great God, in whos Will there is only Order.