The number of reflections in the REFLN list (not the
               DIFFRN_REFLN list) that are significantly intense, satisfying
               the criterion specified by _reflns.threshold_expression. This may
               include Friedel-equivalent reflections (i.e. those which are
               symmetry-equivalent under the Laue symmetry but inequivalent
               under the crystal class) according to the nature of the
               structure and the procedures used. Any special characteristics
               of the reflections included in the REFLN list should be
               described using the item _reflns.details.