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A domain is defined as a collection of logically related observations with a common topic. The logic of the relationship may pertain to the scientific subject matter of the data or to its role in the trial. 

Each domain dataset is distinguished by a unique, 2-character code that should be used consistently throughout the submission. This code, which is stored in the SDTM variable named DOMAIN, is used in 4 ways: as the dataset name, as the value of the DOMAIN variable in that dataset, as a prefix for most variable names in that dataset, and as a value in the RDOMAIN variable in relationship tables (see Section 8, Representing Relationships and Data).

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

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Although the domain name is carefully selected, it is the structures and specifications within the domain that drive placement of data. It is important to note that the domain structure is only used for organizational purposes. The --TEST and --METHOD variable entries in the domain contribute to the identification of the test performed and the conditions under which the test was performed; the domain name or organization is not intended to imply any of this information.

Each domain dataset is distinguished by a unique, 2-character code that should be used consistently throughout the submission. This code, which is stored in the SDTM variable named DOMAIN, is used in 4 ways: as the dataset name, as the value of the DOMAIN variable in that dataset, as a prefix for most variable names in that dataset, and as a value in the RDOMAIN variable in relationship tables (see Section 8, Representing Relationships and Data).

What or who is a subject

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