Or if you want to use an email in more than one program, you would simply include the "send email" flow step within a smart campaign of a program to select the email that resides in Design Studio.  We have all of our operational emails in Design Studio (e.g., contact-us confirmation emails, marketing/sales alerts when MQLs are achieved, etc.).  Same with our email templates.  But campaign emails are typically created as a local asset (based on an email template) within the program itself.