# About internal IDs

Each clinical topic contains an Internal IDs folder. These folders contain unique database identifiers for various study components.

Use the following internal IDs to enhance reports:

- Subject ID—Unique ID that identifies a subject when the Subject Number is not unique. The Subject ID is unique for each subject.
- Form Index, Itemset Index, and Visit Index—Use with repeating forms, items, and visits to report on status information for individual instances of repeating objects.

  Note: Indexes are also known as sequence numbers.

Use internal IDs when you need to identify unique instances of clinical data when there are no other elements containing unique data values.

For example, if the subject numbers in a study are not unique, to uniquely identify a subject in a report, you can use the SUBJECTID element from the Internal IDs folder.

Note: Because internal IDs access database indexes directly, they can improve the efficiency of reports that you create with the Report Studio application. Knowledge of database principles and the Reporting and Analysis database schema is recommended.

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