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The child element SubjectData will mostly be used when the data represents data with e.g. visits and forms, and the information is grouped per subject, whereas the child element ItemGroupData will mostly be used for tabular data, and where the subject information can be in one of the columns of the tables, and the information is not grouped per subject.
The ClinicalData element can however contain a mixture of data grouped per subject, i.e. using SubjectData as well as data that is not grouped per subject, such as tabular data, using ItemGroupData.
Also see the page "See Section 2.7, Clinical Data Keys" for , for a visualization of these possibilities.
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