The number of reflections that satisfy the resolution limits
established by _refine_ls_shell.d_res_high and
_refine_ls_shell.d_res_low and the observation limit
established by _reflns.observed_criterion, and that were used
as the working reflections (i.e. were included in the
refinement) when the refinement included the calculation of
a 'free' R factor. Details of how reflections were assigned
to the working and test sets are given in _reflns.R_free_details.