Specifying the dataFormat argument allows you to format the cell.
For example, "#,##0.00" corresponds to using a comma separator for powers of
1000 with two decimal places, "m/d/yyyy" can be used to format dates and is
the equivalent of 's MM/DD/YYYY format.  To format datetimes use "m/d/yyyy
h:mm:ss;@".  To show negative values in red within parantheses with two
decimals and commas after power of 1000 use "#,##0.00_);[Red](#,##0.00)".  I
am not aware of an official way to discover these strings.  I find them out
by recording a macro that formats a specific cell and then checking out the
resulting VBA code.  From there you can read the dataFormat code.