              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.