Promotional - aka, "Marketing" - emails are just that. An email to promote a product, an offering, an event, content, etc. Whereas operational/transactional emails are used to legitimately complete some sort of engagement/transaction. For example, confirmation/reminder emails when registering for an event; confirmation email after you purchase something; a system-outage notification; etc. None of those emails should be used to secondarily promote something else (unless you're sending to a list of subscribed/engaged users.