Vertical control points
The panorama tools optimizer understands various different schemes for aligning photos using control points: Normal points are t0 points, horizontal control points are t1 points and vertical control points are t2 points.
The main use of vertical control points is to change the perspective of the output panorama such that the marked structures are vertical in the output projection. In a multi image panorama this is only possible if the optimization of Roll and Pitch is allowed for all images and of Yaw for all but the anchor image.
You set vertical control points on a vertical structure as far apart from each other as possible.
Please note that all real world vertical lines only stay vertical in rectilinear, cylindrical and equirectangular projections. In fisheye projection only the vertical line through the image center stays vertical.