Returns an unique identifier for the active sheet. This is a value that is only used by FlexCel, and you can use it together with TExcelFile.GetSheetIndexFromID to retrieve a sheet after deleting or adding sheets. While normally you can use TExcelFile.ActiveSheet to store and retrieve a sheet, if you plan to delete or add sheets before restoring the active sheet, the stored ActiveSheet could become invalid. Note that this is a value internal for FlexCel, and not stored in the xls/x file. SheetIDs will change every time you open the file.
property TXlsFile.SheetID: TGUID