If the Inner Planes are the raw matter and energy that make up the multiverse, the Outer Planes provide the direction, thought, and purpose for its construction. These are realms of spirituality and thought, the spheres where Celestials, Fiends, and deities dwell. The plane of Elysium, for example, isn’t merely a home for good creatures or where spirits of good creatures go when they die. It is the embodiment of goodness, a spiritual realm where evil can’t abide. It is as much a state of being and of mind as it is a physical location.
When discussing anything to do with deities and their realms, the language used must be highly metaphorical. Their actual homes aren’t literally places at all but exemplify the idea that the Outer Planes are realms of thought and spirit.
The planes with an element of good in their nature are called the Upper Planes, while those with an element of evil are the Lower Planes. A plane’s alignment is its essence, and a creature whose alignment doesn’t match the plane’s alignment experiences a sense of dissonance there. When a good creature visits Elysium, for example, it feels in tune with the plane, but an evil creature feels uncomfortable.