A transactional SMS is a message the recipient expects because of something they did: a login code, an order confirmation, a delivery update or a security alert. It is not marketing and may bypass quiet hours.
Transactional messages are triggered by the recipient's own action or by an event on their account. They are time-sensitive, individually addressed and not promotional. Regulators and carriers treat them more leniently than marketing: they do not require the same consent and are usually exempt from quiet-hour rules.
The line is about content, not about how you feel about the message. A receipt that ends with a discount code is promotional. Sending marketing flagged as transactional is the fastest way to have a route closed, because carriers see the pattern.
At SMSMeteor: set window.quiet_hours to bypass on transactional traffic so codes and alerts leave at any hour; the flag is recorded and audited. See the OTP use case.