Identifier to the input data from which the distance restraint is derived.
This data item is a pointer to the _ihm_dataset_list.id in the
This data item may not be applicable for all cases. For example, in case of
ambiguous interface restraints where the interface residues are identified
from multiple experiments, the reference to the _ihm_dataset_list.id is
handled in the IHM_INTERFACE_RESIDUE_FEATURE category rather than here.