In most cases, Workflow manages the relevant data between task automatically, but there are some cases where you need to retrieve the relevant data by program, for instance:
&WorkflowApplicationData = &WorkflowContext.ProcessInstance.GetApplicationDataByName('<RelevanDataName>')
&Variable = &WorkflowApplicationData.NumericValue
&WorkflowContext.ProcessInstance.GetApplicationDataByName('<RelevanDataName>').NumericValue = &Variable
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Where:
&WorkflowContext (WorkflowContext data type)
&WorkflowApplicationData (WorkflowApplicationData data type)
&Variable (in this case Numeric but it could be another type)
Workflow Context
Workflow Process Instance Data Type