Data items in the STRUCT_REF category allow the author of a
               data block to relate the entities or biological units
               described in the data block to information archived in external
               databases.
               For references to the sequence of a polymer, the value of
               the data item _struct_ref.seq_align is used to indicate
               whether the correspondence between the sequence of the entity
               or biological unit in the data block and the sequence in the
               referenced database entry is 'complete' or 'partial'. If
               this value is 'partial', the region (or regions) of the
               alignment may be delimited using data items in the
               STRUCT_REF_SEQ category.
               Similarly, the value of _struct_ref.seq_dif is used to indicate
               whether the two sequences contain point differences. If the
               value is 'yes', the differences may be identified and annotated
               using data items in the STRUCT_REF_SEQ_DIF category.