Checking the Convert Value checkbox opens the formula box. From here, you have the ability to manipulate the returned value with a variety of mathematical possibilities. You can choose common functions from the drop down lists to manipulate the returned value, or you can select the Custom Conversion option.
The Custom Conversion option offers more flexibility when manipulating returned values. This option allows you to use the basic arithmetic operators; +, -, *, /, along with built-in functions for more advanced conversions.
For examples, see Available data transformations.