Completeness (as a percentage) of symmetry-unique
anomalous difference data within the sphere defined by
the diffraction limits (_reflns.d_resolution_high and
_reflns.d_resolution_low) relative to all possible
symmetry-unique anomalous difference data within that
sphere.
In the absence of an anisotropy description this is
identical to _reflns.pdbx_percent_possible_anomalous.