HFWG Recommendation R9:

           On the OGIP data quality flag convention

                    (Approved: 1994 Jul 06)

The meaning and use of quality flags is often specific to a given
instrument and/or data type. However, when devising quality flags for
new FITS files, the HFWG recommends that the integer value 0 (zero) be
used to indicate that the data to which the flag refers is of GOOD
quality and is to be automatically used by downstream software.

An instrument/dataset/application-specific scale of non-zero quality
flag values can be used to indicate why the data is considered of bad
quality, the degree of 'badness', and data flagged to be ignored in
downstream applications by the user.