The universe and starry sky, in fact, lack the concepts of up, down, left, and right. To determine directions, one must select a reference point.
The Boundary River is the reference point for the Four Heavens to determine direction.
The horizontal plane of the Boundary River defines the horizontal direction of Eastern Heaven; while the vertical direction penetrating into the center of the Boundary River is used to determine up, down, left, right, north, south, west, and east.
Entering the Boundary River feels like stepping from Eastern Heaven into another world. A step outside is the vast emptiness of the Eastern Heaven starry sky; a step inside is the tumultuous river where heaven and water meet. Two completely different scenes intersect at the boundary of the Boundary River, bizarre and colorful, giving one a sense of awe.
