IDMS sets:

A Set expresses relationships or association between two record types, where one is subordinate to another. The subordinate record type is called the MEMBER record and the other record type is called as OWNER record. The owner and member relationship is one-to-many. A Set Type defines the structural components or the attributes of a set. A Set Occurrence is an owner record occurrence along with it’s associated member record occurrences. There exists one set occurrence for each owner record occurrence. In case of a NULL set, an owner record may be the only one in the set occurrence if it does not have any associated member records.