The border relm refers to an area of space between the Kelbrid empire, The Andalite people, The Coronzat and The Conglomeration. It is a vast stretch of space that is mostly in between spiral arms of stars, meaning it has few planetary systems. Some say it extends all the way to quadrant 4. Most of the Animorphs search for Ax took place close to or in the border relm. During the Kelbrid invasion the border relm became the Kelbrid's main travel line for military actions into quadrant 1.
The border relm is a very dangerous, damaged place. It is filled with non-dimensional distortions, spatial tears, and warped space fabric, including different temporal anomalies. In some areas, time flows faster, in others slower, and in others it go's backwards. Something catastrophic happened there a long time ago.