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 and vertical control points are t2 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'.