The Sheet where we are working on, 1-based(First sheet is 1, not 0). Always set this property before working on a file. You can read or write this value.
Setting the active sheet will also set the selected sheet in the file, except when the object is a light clone (LightClone) For lightcloned object, the selected file won't change. Also if the file has multiple windows, this property will only change the selected sheet for the first window. Use ActiveSheetForActiveWindow if you want to change the active sheet for a different window.
Finally, remember that a file can have more than one sheet selected, even if only one can be active. To Select mutiple sheets, call SetSheetSelected.
property TExcelFile.ActiveSheet: Integer