The number of reflections that satisfy the resolution limits
established by _refine.ls_d_res_high and _refine.ls_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.