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Custom domains and corresponding specifications may be created based on the three general observation classes described in the SDTM.

New domain specifications may be created when domain specifications in this guide don't exist

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Datasets Other than General Observation Class Domains - SDTMIG v3.4 - Wiki (cdisc.org)

The SDTM includes 4 types of datasets other than those based on the general observation classes:

• Domain datasets with subject-level data that do not conform to 1 of the 3 general observation classes. These include Demographics (DM), Comments (CO), Subject Elements (SE), and Subject Visits (SV), and are described in Section 5, Models for Special-purpose Domains.
• Trial Design Model (TDM) datasets, which represent information about the study design but do not contain subject data. These include datasets such as Trial Arms (TA) and Trial Elements (TE) and are described in Section 7, Trial Design Model Datasets.
• Relationship datasets, such as the RELREC and SUPP-- datasets. These are described in Section 8, Representing Relationships and Data.
• Study Reference datasets include Device Identifiers (DI) and Non-host Organism Identifiers (OI). These provide structures for representing study-specific terminology used in subject data. These are described in Section 9, Study References.

Datasets and Domains - SENDIG v3.1.1 - Wiki (cdisc.org) 

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