Hello valerio,
Hugin is saying that the exposure range between shots may mean you have
bracketed shots that you want to be treated as exposure stacks, but the
differences in exposure may not be constant, and maybe hugin isn't sure if
you really want stacks assigned.
If you want to use exposure stacks then manually assign them, this can be
done in the photos tab (advanced or expert interface).
Hope this helps.
Cheers,
--
Regards,
Terry Duell