To apply value formatting for a data-bound control's content (e.g. for it to be treated as currency, or date-time content), locate this control, click its Smart Tag, and in the invoked actions list, click the ellipsis button for the Format String entry.
Then, in the invoked format string editor, choose one of the predefined formatting styles, or specify a custom one.
To quit the dialog and apply the changes, click OK.
Similarly, when the mail-merge is employed for a control's dynamic content, to apply a value formatting to an embedded data field, select it in the in-place editor, and click the control's smart tag. Then, in the invoked actions list, specify the required format.
Note that when a summary function is being applied to a control's dynamic content, value formatting is applied separately, via the Summary Editor, as described at Add Totals to a Report.
Independently from the general (or, summary) value formatting, you can specify a native XLSX format string, which is to be preserved when the report is being exported to XLSX. This can be done via a control's Xlsx Format String property.
