Straight line control points
The panorama tools optimiser 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 points.
Straight line control points were added later and have a similar effect to horizontal and vertical points, except that the straight line can be at any angle. They have two main uses: Lining up linear features that don't have identifiable detail, such as the edges of a room or overhead cables; and calibrating lens distortion using a single rectilinear photograph of a grid or building.
Although two points are sufficient to define a horizontal or vertical line, more are required for these lines at arbitrary angles. Since the script file format only supports 'pairs' of points, a straight line is generally defined with four, six or eight etc... control points.
Otherwise if you edit a PTOptimizer script and make more than one pair of existing points t3, then the optimiser will try to line them up as a straight line in the output image. You can keep going, with t4, t5 etc... for each group of points that you want in different 'straight lines'.