Sets or returns the ordering of the PivotTableField using an OrderingType. This only has an affect on row labels and column labels.

 public SortOptions.OrderingType Ordering{ get; set; }
Public Property Ordering() As SortOptions.OrderingType

          ExcelApplication xla = new ExcelApplication();
          Workbook wb = xla.Open("template.xlsx");
          PivotTable pt = wb[0].PivotTables[0];
          pt.ColumnLabels[0].SortOptions.Ordering = SortOptions.OrderingType.Ascending;

          Dim xla As New ExcelApplication()
          Dim wb As Workbook = xla.Open("template.xlsx")
          Dim pt As PivotTable = wb(0).PivotTables(0)
          pt.ColumnLabels(0).SortOptions.Ordering = SortOptions.OrderingType.Ascending;

ExcelWriter does not have the ability to perform the actual sorting action. This is handled by Excel when the output is opened in Excel and the PivotTable is refreshed. This property determines how Excel sorts the values for a particular PivotTable field.