When using CellValue to set a cell value, you have to know the datatype you want to enter. That is, if you have a string s:="1/1/2002" and call CellValue[1,1]:=s; The cell A1 will end up with a string "1/1/2002" and not with a date. The same if you have a string holding a number. 

SetCellString tries to solve this problem. When you call SetCellString(1,1,s) FlexCel will evaluate: 

1. If s contains a valid number: If it does, it will enter the number into the cell, and not the string s.

2. If s contains a boolean: If s equals the words "TRUE" or "FALSE" (or whatever you define on the constants TxtTrue and TxtFalse) it will enter the boolean into the cell

3. If s contains a date: If s is a valid date (according to your windows settings) it will enter a number into the cell and format the cell as a date. (see Using FlexCelImport )

4. In any other case, it will enter the string s into the cell.

There is an overloaded version of this method, that lets you specify the original format you want for the cell.

SetCellString Method