A center opening will open from the center and when fully open have panels on both the left and right sides. A right-side stack will open from the left and when fully open will have a stack of panels on the right side. This is commonly used for a sliding glass door with a left side opening. A left-side stack will open from the right and when fully open will have a stack of panels on the left side. This is commonly used for a sliding glass door with a right side opening.