This value of _pdbx_ligscreen_investigation_screening_exp.instance_id identifiers a specific data record, for a given experiment, in the pdbx_ligscreen_investigation_screening_exp category. For example if a liagnd or fragment is characterized by "Electron diffraction microscopy" an experiment will have two data records to link to both PDB and EMDB. Another example, to have both a hit ("On-site binding") and partial-hit ("Off-site binding"), an experiment will have two data records.