How to use the Enable for Calculated Fields checkbox
If the options in the Edit Lookup, Edit Radio and Edit Select windows are numbers you can mark the Enable for Calculated Fields checkbox to use them in calculations. This is especially helpful when using data sets. When this option is selected, calculations can be performed even if the data comes from a data set.
For example, you have an Employee Evaluation form where employees are rated on performance categories using a scale between 1 and 5. The form contains a calculated field that totals the ratings entered per category to help determine any future employee raises and promotions. Formatting rules can also be applied to the selections when this option is marked, as shown in the following illustration.