When working with FlexCel, you always have one active sheet where all commands apply (setting a cell, adding an image, etc.) 

By changing this property, you can modify the sheet you are working on. 

Note that this property is 1-Based, that is, the first sheet is 1, the second 2 and so on until SheetCount 

Read this property to get the active sheet, write this property to set it. 

The following code will write a value in cell a1 in the first sheet, and then other in the second sheet. 

FlexCelImport.ActiveSheet := 1;
FlexCelImport.CellValue[1,1] := 'This cell is in the first sheet!';

FlexCelImport.ActiveSheet := 2;
FlexCelImport.CellValue[1,1] := 'This cell is in the second sheet!';

TFlexCelImport Properties