The sequence index for the second monomer partner in the cross link.
This data item is a pointer to _entity_poly_seq.num in the ENTITY_POLY_SEQ category
and the _ihm_cross_link_restraint.seq_id_2 in the IHM_CROSS_LINK_RESTRAINT category.
Data type code
Data type detail
int item types are the subset of numbers that are the negative
or positive integers.