It is the orientation of the plane, but not the orientation inside the plane. How does BSP decides in which direction it must display the textures inside a plane?
The BSP file stores faces, which define a number of edges (or vertices). Faces are used in rendering, and contain the texture being drawn.
Faces also refer to a plane and a “side”. If the side is 1, the normal of the plane is flipped which has the effect of using the back side of the plane.