InfoPath allows you to designate a field as required several different ways. One of the easiest ways to designate that a field is required, is to look at the properties of the field. In the properties dialog, just simply check the “Cannot be blank” checkbox. This action will require the user to supply some data in the field. Visually it will also place a red asterisk in the control, to tell the user this field needs some data in it.
Either solution works fine if you are only requiring that specific field. But what if you needed to require an additional field, if something else was selected on the form? At the time of designing the form you would not know what the user will select. The following article will show you how to conditionally require a field based on the value of another field. The form is basic, with just a simple radio button and a text box. If the radio button answer is “Yes,” we need to require some value in the text box. If the radio button answer is “No,” the user can continue.
The rule you need to add for this solution is a Validating rule. Add the Validating rule with which field you want to make the decision on, and another rule to make sure the conditional field is not empty.