This trigger starts a workflow whenever a subscriber is added to a list. You can also run a workflow whenever a subscriber gets removed from a list.
It doesn’t matter “who” adds the subscriber to a list, the workflow will always start if the lists match.
It is similar to the Form Conversion Trigger but doesn’t require a form submission.
Choose all the lists you like to start the workflow when a subscriber gets assigned to them.
Learn more about Triggers here.