The user needs to specify what are the keywords to look for in each frame header to get the offset information in x and y, in pixels. NTT offsets as found in SOFI data have been shown to be reliable to subpixel accuracy in the best cases, usually in the range of about 5 pixels in error, and never more than 10.
Differences are mainly due to the optics and wavelength range.
The default search area in this case is set to pixels in x and y, which should cover more than really needed.