So x, y, and z are where the magic is being cast, and <I> is what spell is being cast?  Or is <I> where the magic is being stored, and x, y, and z is where it's being moved to?

Or is it more <I> the same dimension, but slightly shifted, so if your x, y, and z are at your house, then you just change your <I> slightly, you're outside your house?