In Spread, there are several objects that should not be confused. The sheet in a multiple-sheet workbook corresponds to a SheetView object. In a hierarchical data display, each child (expanded) sheet of the parent sheet corresponds to a separate SheetView object. The FpSpread object is somewhat like a workbook.
For more detailed information on these objects, refer to the FarPoint.Web.Spread.SheetView object and the FarPoint.Web.Spread.FpSpread object in the API reference documentation.