Note: If you created a segment email that will be sent out only once (e.g a newsletter), you need to add an Unpublish date to stop it from sending out to future members of the segment. Set it to 2 hours after you plan to send it out.
A segment email can only be sent to a segment (list), and cannot be used in automations. Similar, a template email can only be used in automations (campaigns). You can, of course, send the template email to a segment within the automation, but if that's the only action you want to execute, we recommend using a segment email instead.