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
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.