Dragging a numeric field directly onto the body of a report automatically creates an expression using the Sum function. Since the scope of a report item directly on the body of the report is the entire dataset, this yields a grand total for the report.
You can also add grand totals and subtotals to groups in any of the following data regions:
Subtotaling is generally accomplished using the Sum function in an expression, along with the scope parameter. The expression uses the default scope, generally the group level in which the report item containing the expression resides, if one is not specified. For more information on scope, see the Functions topic.