To display reminder messages or alerts to users as they interact with forms in ServiceNow, Client Scripts are used. Specifically, an onSubmit Client Script can be configured to display a message or perform validation just before the form is submitted by the user. This script runs on the client (browser) and can prevent submission or prompt the user with informational messages.

Business Rules execute on the server and cannot directly interact with the user interface in real-time. Policies and Data Policies enforce data consistency but do not provide user messages or reminders during form submission.

Therefore, the Client Script is the correct mechanism to display a reminder message dynamically as the incident is submitted.