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All relationships make use of the standard domain identifiers STUDYID, DOMAIN, and USUBJID. In addition, the variables IDVAR and IDVARVAL are used for identifying record-level merge/join keys. These keys are used to tie information together by linking records. The specific set of identifiers necessary to properly identify each type of relationship is described in detail in the following sections. The following are examples of variables that could be used in IDVAR:

• The sequence number Sequence Number (--SEQ) variable uniquely identifies a record for a given USUBJID within a domain. The variable --SEQ is required in all domains except DM. For example, if a subject has 25 observations in the Vital Signs (VS) domain, then 25 unique VSSEQ values should will be established populated for this subject. Conventions for establishing and maintaining --SEQ values are applicant-defined. Values may or may not be sequential depending on data processes and sources.the subject.
• The Reference Identifier The reference identifier (--REFID) variable can be used to capture a sponsoran applicant-defined or external identifier, such as an identifier provided in an electronic data transfer. Some examples are lab-specimen identifiers and ECG identifiers. --REFID is permissible in all general observation-class domains, but is never required. Values for --REFID are sponsor-defined and can be any alphanumeric strings the sponsor chooses, consistent with their internal practices. 
• The Grouping Identifier The grouping identifier (--GRPID) variable, used to link related records for a subject within a domain, is explained in Section 8.1, Relating Groups of Records Within a Domain Using the --GRPID Variable.