Bubble allows custom workflows to reuse the same logic in more than one workflow. This event defines that workflow. Name the workflow, and then use a 'Trigger a custom event' action to run this workflow from another workflow. Think about this feature as 'functions' in Bubble.
Enter the name of the workflow. Select custom workflows in the 'Trigger a custom event' action by name.
If you want the custom workflow to take a thing as an argument, i.e., an entry from the application database, define the type here. If this field is set, the 'Trigger a custom event' action prompts you to define which thing should be sent when running the workflow.