How to code an Event Subprocess

Hello,
In the scenario below, I understood the reply, but could you explain me how to implement an "Event Subprocess" please ?

I don't see any simple option or such smart service in the process modeler palette. And in the "subprocess" service I don't see how it can be triggered by an event.

Do I need a Rule or Timer event, or "Receive Message" to activate the supProcess?

If you have a tutorial on internet, it would be perfect :-)

Ms. Durant is working on an Appian application to track customer orders for a retail company. The application needs to trigger a series of tasks to validate order details, check inventory, and process payments. However, if the payment fails, the task flow should automatically halt and notify the customer service team to follow up. Additionally, the process must be able to send notifications to the customer about their order status at each stage.
What is the best way for Ms. Durant to design the process model to ensure that payment failures are handled efficiently, and notifications are sent throughout the workflow?

=> Use an "Event Subprocess" to handle the payment failure as an exception, and configure "Send Notification" nodes in the process to notify customers at each stage.