Frames Alignment

Usually alignment based on a single star (without frame rotation) is adequate but if pictures taken on different nights are to be combined, two star based alignment is required (with frame rotation) as probably telescope polar alignment or camera position are not identical on both nights. You must also use this technique if there is field rotation due to polar misalignment or using azimuth mount.
Some programs allow you to manually select stars to be used as reference stars and others just do it selection automatically.
Some programs like Registax and DeepSkyStacker divide the image into several zones and each zone is separately aligned.
Personally I prefer the alignment based on a single manually selected star.