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 test reflections (i.e. were excluded from 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.