An analytical rectangle is defined by three points. Opposite edges are always parallel to each other. The fourth corner point is computed from the given three points internally.
The volume defined by a rectangle is an infinite half space, trimming out the other side of the plane. Which one of the sides is 'inside' depends on the winding order of the points. The interior side is defined by the surface normal computed using the right hand winding rule.
For example, one can use a rectangle object to split an object into two parts using the Boolean operations.