I suppose something that carries some operational information and/or that is worth to be tracked, like "you've subscribed to our service", "your order is ready for shipping", "this is your password reset link", "your credit card has been charged.." etc opposed to mails like "hi Deb, how was the party last night?" that are merely informative / casual conversation (of course nothing stops you from sending this kind of mails with "transactional mail providers" and keepin track of relative metrics :))

Another distinction is that you typically send transaction email to a single person, because some transaction has happened. Bulk email you send a (perhaps customized) email to an entire list of recipients in one go.