Completeness (as a percentage) of symmetry-unique data
               within the spherical shell defined by its diffraction
               limits (_reflns_shell.d_resolution_high and
               _reflns_shell.d_resolution_low) relative to all
               possible symmetry-unique reflections within that shell.
               In the absence of an anisotropy description this is
               identical to _reflns_shell.percent_possible_all.