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• Explicit relationships, (e.g., concomitant medications taken as a result of an adverse event); or
• Information of a nature that necessitates using multiple datasets and that may need to be examined together for analysis or proper interpretation.  

It is not necessary to use the RELREC dataset to identify associations from data in the Supplemental Qualifiers (SUPP--) datasets or the Comments (CO) dataset to their parent general-observation class dataset records or special-purpose domain records, as both these datasets include the key variable identifiers of the parent record(s) that are necessary to make the association.

RELREC will not be used to represent relationships already represented in Supplemental Qualifier (SUPP–) datasets for nonstandard variables or Comments (CO) for for unstructured free-text.

Subject records or datasets expressing a relationship are specified using the key variables STUDYID, RDOMAIN (the domain code of the record or dataset in the relationship) with IDVAR (the variable that identifies the related record(s)). Each record in the RELREC special-purpose dataset contains keys that identify a record (or group of records) and an identifier for the relationship which is stored in the RELID variable. The value of RELID is chosen by the applicant and it is recommended that applicants use a standard system or naming convention for RELID (e.g., all letters, all numbers, capitalized).

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Relationships between datasets are represented using a single record for each related dataset that identifies the key(s) of the dataset that can be used to relate the respective records. Relationships between datasets are specified using the key variables STUDYID, RDOMAIN, and IDVAR only. The values of variables USUBJID and IDVARVAL will be null as relationships between specific subject records will not be identified. The variable RELTYPE will identify the type of relationship between the datasets (e.g., a one-to-one or parent-child relationship). The allowable values for RELYPE are ONE and MANY per CDISC Controlled Terminology. This information defines how a merge/join would be written, and what would be the result of the merge/join. The possible combinations are: