Relationships between elements of two documents that are recorded in a NIST IR 8278A-compliant format and shared by the OLIR Catalog. There are three types of OLIRs: concept crosswalk, set theory relationship mapping, and supportive relationship mapping.
Sources:
NIST IR 8278Ar1
under Online Informative Reference
NIST IR 8278r1
under Online Informative Reference