The Rf(x) column is only available for selection groups and selection alternatives if you use variables in the Product Configurator.
Under the f(x) button in the column there is a formula window where you can create formula rules based on the variables entered in the procedure. The formula rules are used for excluding selection groups and excluding/including selection alternatives in the configuration window, based on the values entered in the variables. In the Formula rule column to the right you can see the formula entered in the formula window.
For further description of how to use formulas, see the Formula window section in the Product Configurator chapter in the Using MONITOR Procedures guide.