The number of reflections that satisfy the resolution limits
established by _refine_hist.d_res_high and
_refine_hist.d_res_low and the observation limit
established by _reflns.observed_criterion, and that were used
as the working (i.e., included in refinement) reflections when
refinement included 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.