Rules are a great way to make a form that is based on your form template perform an action without your having to write a single line of code. You can use rules to perform tasks, such as setting the value of a field, getting data from an external data source, or submitting a form. Microsoft InfoPath Team Blog: Best Practices in Rules provides a list of the best ways to use rules — this is straight from the people who created the InfoPath rules.
For more information about rules, see the following articles:
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