I understand how I can communicate with Excel
from with a Jscript (see example below.
But where is the documentation (API) of what messages I
can send to such an instance ExcelSheet

var ExcelSheet;
ExcelApp = new ActiveXObject("Excel.Application");
ExcelSheet = new ActiveXObject("Excel.Sheet");

// Make Excel visible through the Application object.
ExcelSheet.Application.Visible = true;
// Place some text in the first cell of the sheet.
ExcelSheet.ActiveSheet.Cells(1,1).Value = "This is column
A, row 1";
// Save the sheet.
ExcelSheet.SaveAs("C:\\Temp\\TEST.XLS");
// Close Excel with the Quit method on the Application
object.
ExcelSheet.Application.Quit();