From the Game's Introduction:
The GRAPE Grimoire is a description of possible magic systems for use in GRAPE. They describe possible rules of reality to govern magic in a campaign world where magic exists. These systems are usable in various combinations, in various genres. Many classic magic-using characters combine two or more of these magic systems.
In most games, the GM works to keep the game "realistic" - which means that s/he must make rulings that allow the characters' lives to make sense within the context of the physical laws. By adding magic to a campaign, the Game Master has decided to add in laws of nature which do not exist. S/he is advised to think through exactly how those laws of nature work, and the effect which that will have on the game world, well in advance.