Horizontal 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, vertical control points are t2 points, and straight line control points are t3, t4, etc... points.
The main use of horizontal control points is to change the perspective of the output panorama such that the marked structures are horizontal 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 horizontal control points on a horizontal structure apart from each other. However, be careful in a cylindrical or equirectangular panorama not to set them 180° apart, since this won't level the horizon.
Please note that all real world horizontal lines only stay horizontal in rectilinear[*] projection. In cylindrical, equirectangular and fisheye[*] projections only the horizon itself stays horizontal.