The number of reflections that satisfy the resolution limits
established by _refine_ls_shell.d_res_high and
_refine_ls_shell.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.