Observations about study subjects are represented in a series of domain datasets for which there will generally be a single dataset for each domain. All datasets are structured as flat files with rows representing observations and columns representing variables. Each domain dataset will named a unique, 2 to 4 letter 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. The SDTM lists only the name, label, and type, with a set of brief CDISC guidelines that provide a general description for each variable.
Datasets and Domains - SDTMIG v3.4 - Wiki (cdisc.org)
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