Action Rule is the most frequently used Rule in InfoPath forms where you want to perform any action on control of the form or on form load or want to submit the data. Also, on firing the action, you can show the Validation Error message.
SharePoint – InfoPath Form
Microsoft SharePoint InfoPath form provides some types of Rules to perform the action on the controls of the form. Validation Rule is one on them. InfoPath form automatically validates the data you entered and if does not match with the required condition, it fires the error message.
InfoPath form is the SharePoint feature provided to create form without any HTML and CSS code. InfoPath forms are created by the InfoPath Designer tool.
SharePoint Form library is the collection of XML forms (InfoPath forms). SharePoint Form library available in the Site Contents of the site used to create/save/update the InfoPath forms.