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The OrderNumber attribute provides an ordering on the ItemGroupDefs (within StudyEventDef or ItemGroupDef) for use whenever a list of ItemGroupDefs is presented to a user. Order of execution is preferably defined in a workflow (element see Section 3.2.2.1.8, WorkflowDef) but when used without a workflow, may be used the define the order in which data entry forms are presented in an application UI.
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