Understand on update of a field
The script actions that you define in the On update of a field event will be executed when a field value is updated or modified and the form with the changed field value is successfully submitted to the database. So the workflow is triggered after the form submission
Let's assume you have an Incident Management application that is used to register, track, and resolve incidents or issues in your business. In this application you have an IncidentDetails form that has information on the issue, the details of the user who raised the issue, the support representative who has been assigned the incident, and the status of the incident. The Incident Status field is a choice field with Opened, Assigned, In Progress, On-hold, and Resolved as options. You can write script for on update of this field. If the Status is changed, you can write a script to send an email notification to all the email address fields included in the form, providing information that the status has been updated.