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.