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When you design a form template in InfoPath, you can control what type of data people are allowed to enter in each field of the form. For example, you might want a field for telephone numbers to be exactly ten digits, including a three-digit area code in parentheses.
By using data validation, you can check the data that people put in forms, and then display various messages if the data does not match conditions that you specify. Watch the demo to see how easy it is to configure data validation for your form templates!
How to do it (text version):