This value is False by default to maintain backward compatibility.
When True, format strings will be validated when a IWorksheetCellFormat.FormatString property is set. An invalid format string will cause an exception. When False, invalid format strings will be allowed, but if the display text of a cell is requested, an exception will be thrown at that time. If invalid format strings are allowed and the workbook is saved and opened in Microsoft Excel, it will show an error.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2