I cannot speak to Nebulosity's terms for alignment, but generally, there are three ways to align.
Translate: This moves a star up, down, left or right. The movement is 90 degrees from the
top or bottom edges of the frame.
Rotation: This moves the whole field around. In other words, a star can move in a curve, not just at 90 degrees to the edge of the frame.
Scale: The frame itself can be stretched inward and outward. (Grown or shrunk)
In practice, all three of these can be used at the same time. Furthermore, in sophisticated software, the individual centroids of the various stars can be used so that the corners, for instance, are scaled, rotated, or translated differently from the center of the frame.
All this is done by using a master frame as where the stars should be, and having the stars of the other frames properly aligned to that one master frame.
Edited by Alex McConahay, 23 September 2019 - 12:47 PM.