The value of _pdbx_diffrn_batch_scan.batch_id must
               uniquely identify the association between the derived
               quantities (PDBX_DIFFRN_BATCH category) and
               experimental scan information (DIFFRN_SCAN category).
               This is a pointer into the PDBX_DIFFRN_BATCH category
               (pdbx_diffrn_batch.id item).