Afit requires to read an input file with starting parameters. Its name is fxx.par where "xx" il the function number (e.g. f15.par for line+sine fitting).
Normally the fxx.par has a more explicative name such as f15-0414.par (referring to the NOAA global temperatures dataset relative to April 2014); that name will be changed to f15.par when necessary. The output file is, by default, afit.out, which will be moved to, e.g., afit15-0114.out.
Bongo can run afit by means of the command "DOS", like in this part of a Bongo command file.