Selection of alignment points

[Guest]

I've used both Registax and Autostakkert for stacking pictures of the surface of the moon with some success. I wondered what criterion they use when you ask them to select the alignment points. Almost always you can see that many of the points appear to have no discernible feature. I would have expected that they would select points where there's a distinct gradient. First of all why would they pick points like that and secondly how can it align points on two separate images when there is no feature there to align to -  if you see what I mean?

Cheers

Steve