A cosm is a reality, a dimension which is physically separate from other worlds and other realities. Each character has a home cosm, which is where he is from, and which describes the sort of reality he is used to living in. Core Earth is the cosm of Earth, the Earth not under the control of a Possibility Raider — the Earth we are used to. Each of the other realms on Earth is part of a cosm under the control of a Possibility Raider, called a High Lord.
Characters not from Core Earth are presumed to have had good reason to quarrel with the High Lords or their minions; being a possibility-rated character (having the reality skill on your template, aka a Wild Card) is reason enough for someone connected to a High Lord to take notice of you.
Home Cosm Axiom Levels
Players should record the axiom levels of their home cosm on their character sheets.