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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).

Data represented in SDTM datasets include data as originally collected or received, data from the protocol, assigned data, and derived data. 

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

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